Currently im storing the date of birth of a user in milliseconds (since 01.01.1970). Do i have to ensure that it is the same date of birth in every country or is it common that it can happen that it is the 11th November in country X and the 12th November in country Y?
If instead it is common practice to have the exact same date of birth in each country, how could i ensure that the milliseconds i store always point to the exact same day of month in each country?
Few people know the time-of-day of their birth. So, no, that level of detail is not commonly tracked. You never see time-of-birth and birth-time-zone on passports and such. They track a date-only value, without time of day and without time zone.
Bartenders, border control agents, etc. use the local current date to calculate age, with no attempt to consider time of day or adjust for time zone. Consider that partial-day difference to be truncated, ignored.
To adjust a moment accurately from one time zone to another, you need a date and a time-of-day. Without a time-of-day you have perhaps 23-25 hours of possible moments when their birth may have occurred.
For example, a birth that occurs a few minutes after midnight in Paris (1 or 2 hours ahead of UTC) on the 24th is still “yesterday” the 23rd in Montréal (4 or 5 hours behind UTC). But if that birth occurs at 06:00 in the 24th, then the date is the same (24th) in both Paris & Montreal, but is “yesterday” (23rd) in Vancouver BC and Seattle where the clocks are 7 or 8 hours behind UTC.
In SQL use a data type akin to the standard DATE type.
When generating string representations of date-time values, use the formats defined in the ISO 8601 standard such as
1960-07-11. For date without year use
--07-11. The java.time classes in Java use ISO 8601 formats by default when parsing and generating Strings.
If for some reason you are forced to put a date-only value into a date-time field, then you must set some arbitrary time-of-day. Noon is one possibility. I suggest setting the time portion to the first moment of the day.
Be aware that the first moment is not always
00:00:00. Daylight Saving Time (DST) and perhaps other anomalies affect midnight in some time zones. So first moment of the day might be
01:00:00 for example. Java can determine the first moment of the day appropriate to a particular time zone.
ZoneId zoneId = ZoneId.of( "America/Montreal" ); LocalDate today = LocalDate.now( zoneId ); ZonedDateTime startOfToday = today.atStartOfDay( zoneId );