com.smartgwt.client.docs

## Interface DateFormatAndStorage

-     public interface DateFormatAndStorage

  ### Date and Time Format and Storage

  The Smart GWT system has the following features for handling Date and Time type values within DataSources and databound components.

  DataSources and databound components may define fields of type `date`, `time`, or `datetime`.

  ### "date" handling

  Fields of type `date` are considered to be logical Dates with no time value, such as a holiday or birthday. In the browser, values for "date" fields are stored as Date objects, but when formatted for display to the user, they are typically displayed without any time information.

  When using the Smart GWT server framework, "date" values are automatically transmitted with year, month and day preserved and time value ignored.

  When sent or received in XML or JSON, date field values should be serialized in the XML Schema date format - `YYYY-MM-DD` - are expected to be received in the same format. Any time value present for a "date" field is ignored.

  The DateUtil.createLogicalDate() method may be used to create a new Date object to represent a logical date value on the browser.

  System wide formatting for dates may be controlled via the `DateUtil.setNormalDisplayFormat()` and `DateUtil.setShortDisplayFormat()` methods.

  ### "datetime" handling

  Fields of type `datetime` are dates with full time information. In the browser, values for datetime fields are stored as Date objects.

  When using the Smart GWT server framework, "datetime" values are automatically transmitted such that the resulting Date object has the same GMT/UTC timestamp. This value is referred to elsewhere in these discussions as an "epoch value", and it is precisely defined as: the number of milliseconds since midnight on January 1st 1970 in the UTC timezone (because 1970-01-01 00:00:00 UTC is "the epoch").

  To ensure that "datetime" values are persisted on the server with full millisecond resolution (rather than only seconds), you may need to set `DataSourceField.storeMilliseconds`.

  When sent or received in XML or JSON, datetime field values should be serialized out as full datetimes using the standard XML Schema datetime format (EG:`2006-01-10T12:22:04-04:00`). If no timezone offset is supplied, the value is assumed to be GMT/UTC.

  System wide formatting for datetimes may be controlled via the `DateUtil.setShortDatetimeDisplayFormat()` method. Datetimes will be displayed to the user in browser local time by default (see also timezone notes below).

  ### "time" handling

  Fields of type `time` are time values in the absence of a day, such as the beginning of the workday (9:00). In the browser, values for "time" fields are stored as Date objects with the time in browser local time. The date information has no meaning and only the time information is displayed to the user.

  Time formatting is handled by the String class APIs.
  When using the Smart GWT server framework, "time" values are automatically transmitted such that the resulting Date object has the same hour, minute and second values in local time, and year/month/day is ignored.

  When sent or received in XML or JSON, date field values should be serialized as hours, minutes and seconds using the standard XML Schema time format - `"22:01:45"`. Timezone is not relevant and should be omitted.

  The DateUtil.createLogicalTime() method may be used to create a new Date object to represent a logical time value on the browser.

  ### Timezone settings and Daylight Savings Time

  By default, "datetime" values will be shown to the user in browser local time, as derived from the native browser locale. Developers may modify this behavior by specifying an explicit display timezone via String.

  Note that depending on the specific date being displayed, a Daylight Savings Time offset may also be applied based on the browser locale. To disable this behavior set String to false.

  If a custom timezone is specified, it will be respected by all `TimeDisplayFormat`s, and by the standard short `DateDisplayFormat`s when formatting dates representing datetime type values. However native JavaScript Date formatters, including `toLocaleString()` will not respect the specified timezone. Developers specifying a custom timezone may therefore wish to modify the `DateUtil.setNormalDisplayFormat()` to avoid using a native JS Date formatter function.

  Note that in addition to the system-wide date, datetime and time-formatting settings described above, databound components also support applying custom display formats for date values. Typically this can be achieved via a custom `dateFormatter` or `timeFormatter` at the field level (see `DataSourceField.dateFormatter`, `DataSourceField.timeFormatter` and for example `ListGridField.dateFormatter`). Date formatting may also be configured at the component level by setting the `dateFormatter`, `datetimeFormatter` and `timeFormatter` attributes (See for example `ListGrid.dateFormatter`, `ListGrid.timeFormatter`, and `ListGrid.datetimeFormatter`).

  ### Database storage of datetime values by Smart GWT Server

  Support for timezones and datetime values varies across the database vendors. There is a standard approach proposed by the SQL:1999 standard, but as is often the case with SQL standards, support is far from universal and there are many idiosyncrasies. This leads to a situation where different databases offer different ways for an internationalized application to store datetimes, and some offer facilities that others lack. For example, PostgreSQL supports the SQL:1999 standard TIMESTAMP WITH TIME ZONE syntax; MySQL allows you to achieve the same end by providing proprietary methods to set a timezone globally or per-connection; Informix has no explicit timezone support at all. For these reasons, Smart GWT Server implements framework-level support for difficult datetime issues like timezones and Daylight Saving Time, and does not rely on any native database features.

  #### Database storage of datetimes in a nutshell

  1. Smart GWT stores datetime values by formatting them as a string, and retrieves them by asking the database to format as a string. Smart GWT uses the same timezone for formatting and for reading from the database, avoiding any shift in the datetime value from storing and retrieving the date.
  2. Smart GWT is unaware of the actual timezone used by the database, and does not specify a timezone when storing or retrieving values. Depending on your database settings, your database may be interpreting the datetime strings stored by Smart GWT as server local time, UTC, or another timezone. Thus, if you use JDBC directly or a SQL querying tool to read datetime values from your database, the value you get may be different from what is retrieved via Smart GWT. If it's important to make these values match, you can configure Smart GWT and your database to read and write using the same timezone - see the more detailed discussion below

  #### Database storage of datetimes in detail

  Every database supported by Smart GWT handles datetimes as human-readable strings of text like '2012-05-13 16:48:20' (in terms of the SQL interface, that is - doubtless many of them implement datetimes internally as epoch values). Of course this is perfectly reasonable, but viewed globally it is also vague unless you specify a timezone, and support for doing that varies across the database vendors as described above. If you are using the SQL storage engine built in to Smart GWT Server (Pro or better only), uniform support for the storage of datetime values in database tables is provided at the framework level.

  As previously discussed, datetime values are transmitted between client and server by encoding and decoding in UTC, ensuring that there is no shift in the datetime value. If the browser and server are in different timezones, Date.toString() on the server will show a different date and time than you see in the browser, but the underlying datetime value is the same.

  Then, for storing to a database, Smart GWT's storage behavior is governed by the `server.properties` flag `sql.{database-name}.useUTCDateTimes`. For example:

            sql.defaultDatabase: AppDatabase
            sql.AppDatabase.database.type: mysql
            sql.AppDatabase.driver: com.mysql.jdbc.jdbc2.optional.MysqlDataSource
            sql.AppDatabase.useUTCDateTimes: true
            etc

  If the `useUTCDateTimes` flag is set, Smart GWT generates SQL that renders datetimes as UTC values; otherwise it generates SQL that renders datetime values in the JVM's local timezone. This process is reversed when datetime values are fetched back out of the database, so values round-trip correctly.

  To aid understanding, consider this round-trip example. We have a user in New York and a user in London; the server is in San Francisco and is configured to use the US Pacific timezone (this is mentioned explicitly because servers are often configured to use UTC, wherever they are in the world). Our New York user saves a record representing an online meeting to take place on May 23 2016 at 2pm local time. This is what happens:

  - Client code uses local time '2016-05-23 14:00:00' in the user's local timezone (EDT, US Eastern Daylight Time, which is GMT-0400) to derive the epoch value 1464026400000, which also represents '2016-05-23 18:00:00' UTC (as you would expect, since EDT is GMT-0400)
  - If the `useUTCDateTimes` flag is set, the server renders this epoch value as a UTC time when generating the SQL. It is important to understand that this concept of the datetime being UTC is something we are doing at the framework level; the database may think it is storing a local datetime, or a UTC datetime, or it may not consider the issue of timezones at all. The important thing is, we told it to insert '2016-05-23 18:00:00', so we can reasonably expect that this is the value we will get out of it when we read the row back
  - If the `useUTCDateTimes` flag is not set, the server renders this epoch value as a time in the JVM's local timezone when generating the SQL. Again, keep in mind that the concept of the datetime being in a particular timezone is something we are doing at the framework level; as above, the important thing is, we told it to insert '2016-05-23 11:00:00' (because it is May so San Francisco is on Pacific Daylight Time, which is GMT-0700), so that is what we will get back when we read it (there are cases where this may not be true; see the section "Store using UTC or local timezone?" below)
  - Now our user in London comes along and reads the same record. The row is read and an epoch time is derived, by treating the value read from the database as either a UTC datetime or a time in the JVM's local timezone, depending on the `useUTCDateTimes` flag (in fact, it isn't always that straightforward behind the scenes because some databases/JDBC drivers perform automatic conversions on datetime values that cannot be suppressed, but Smart GWT Server compensates for such cases)
  - The UTC epoch value delivered from the server is now used to create a Date object on the client; built-in datetime converters and/or Smart GWT facilities like the String will be used to derive a display value applicable to the London user, for whom the equivalent datetime is '2016-05-23 19:00:00' (London is on British Summer Time, GMT+0100, in May)
  As the above example shows, the process is actually fairly straightforward. However, you need to consider that it is the Smart GWT framework that ensures that datetimes coming out of the database are converted using the same timezone that was used when they went into the database. Any non-Smart GWT systems that read the same database tables will have to do any necessary conversions manually.

  ### Store using UTC or local timezone?

  As described above, Smart GWT Server can be configured to store datetimes using either UTC or the JVM's local timezone. On the face of it, there is little to choose between the two: Smart GWT will ensure that values are consistent across server and client regardless of which you choose. However, we recommend that you choose UTC as your storage strategy, for two compelling reasons

  - Because storage using the JVM's local timezone just uses whatever the local timezone happens to be at the time of writing or reading, changing the operating system or JVM timezone will change the meaning of the datetimes already stored.For example, if you change your server's timezone from US Eastern to US Pacific, any datetimes you have already stored will effectively be shifted forward by three hours. This does not happen if you choose UTC as your Smart GWT storage strategy, because it is independent of server timezone
  - As most people know, Daylight Saving Time is a scheme used by many countries worldwide to maximize the number of usable hours of daylight in the summer months. This involves shifting clocks forward in Spring (typically by one hour, but other values are used) and then back again in autumn. This leads to two different but related problems if you choose to use any ordinary local timezone for storage of datetimes
    - In Spring, there is a missing hour, usually the hour between 1am and 2am on the day of transition to DST. If you are using a real local timezone for storage of datetimes, it is not possible to record any datetime value occuring within that hour
    - In Autumn, there is an extra hour, usually created by setting the time back to 1am when 2am is reached on the day of DST transition. If you are using a real local timezone for storage of datetimes, it is not possible to unambiguously record any datetime value occurring within that two-hour period
    Because UTC is a fixed reference point and not a "real" timezone, DST is not relevant and all datetimes can be recorded unambiguously without exceptions or edge cases.

  ### Troubleshooting Date and Time values

  Date and time storage and timezones can be confusing, and Isomorphic receives a steady stream of false bug reports from users that are incorrectly analyzing logs and diagnostics. Please consider the following points when troubleshooting issues such as date values changing to a different day, or datetime value shifting when saved and reloaded:

  #### 1. compare values for "datetime" fields via date.getTime()

  Whenever you use Date.toString() (client or server-side) the value you get is based on the server or browser timezone.

  Perhaps you are troubleshooting an issue with datetimes and you try to log the value of a Date like this:

           Date someDate = <some expression>;
           log("date value is: " + someDate);

  Code like this will show the datetime value in the server's timezone if executed server-side, and in the client's timezone if executed client-side. If they are in different timezones, the hour or day will be different, whereas the actual datetime value - milliseconds since epoch as retrieved by Date.getTime() - is the same. To correctly compare two datetime values, compare the result of getTime().

  #### 2. "date" and "time" field values cannot be compared via getTime()

  This is the inverse situation as for "datetime" values. As explained above, "date" values have no meaningful values for time fields (hours/minutes/seconds) and "time" values have no meaningful values for date fields (month/day/year). Here, the result of Date.getTime() is not meaningful, and values should be compared via getHours(), getMonth() et al.

  #### 3. the display timezone does not affect Date.getHours(), Date.getDay() et al

  If you've called setDefaultDisplayTimezone() to cause all datetime values to be rendered in a particular timezone, this does not affect the return values of Date.getHours(), which will still return values for the browser's current timezone. Hence it is not a bug if you have a "datetime" value which is displaying as 4am, but getHours() returns 10 or some other number. This just reflects the timezone offset between the timezone passed to setDefaultDisplayTimezone() and the browser's local timezone.

  #### 4. use correct DataSourceField types and use the matching FormItem type

  If you declare a field as type "date" but values you provide actually contain specific hours, minutes and seconds, these will not be preserved. The system will discard or reset the hours, minutes and seconds in the course of serialization or editing. Likewise if you declare a field as type "time" but actually provide values where year, month and day have meaning, these values will be dropped.

  Similarly, DateItem expects values for "date" fields, TimeItem expects values for "time" fields, and DateTimeItem expects values for "datetime" fields. Providing the wrong type of value to a control, such as providing a value from a "datetime" field to a DateItem, will have unspecified results.

  If you want to take the date and time aspects of a "datetime" value and edit them in separate FormItems, use `getLogicalDateOnly()` and `DateUtil.getLogicalTimeOnly()` to split a datetime value into date and time values, and use `DateUtil.combineLogicalDateAndTime()` to re-combine such values. Otherwise it is very easy to make mistakes related to timezone offsets.

  #### 5. check data at every phase when troubleshooting

  If you're having a problem with round-tripping "datetime" values or "date" values shifting to another day, you need to isolate the problem to a specific layer. Bearing in mind the techniques above for comparing values, you potentially need to look at any/all of the following:

  1. what value do I have on the server-side before it's sent to the client?
  2. what value is being transmitted to the client? (use the RPC Tab of the Developer Console to see the actual data sent)
     - was the value shifted to a different time/date by my serialization approach?
     - does it have the right format? (see above for correct JSON/XML formats)
  3. what value do I have on the client before it gets to any widgets (eg, do a direct call to `DataSource.fetchData()` and inspect the data in the callback)
  4. what value does the FormItem or other editing widget report before saving is attempted?
  5. what value is reported right before the value is serialized for transmission to the server (`DataSource.transformRequest()` is a good place to check)
  6. what value is being transmitted to the server? (use the RPC tab - same concerns as for server-to-client transmission above)
  7. what value does the server have after de-serialization, before saving to the database or other permanent storage?
  8. what value is sent to the database or permanent storage? If generating SQL or another similar query language, does the value in the SQL statement include an explicit timezone? If not, how will the database interpret it?