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Using now() in different time zones (or with daylight saving time)

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Neil Rogers
Neil Rogers on 6 Dec 2019
Edited: Neil Rogers on 9 Dec 2019
If two people in two different time zones simultaneously used the now() function, would they get different answers? Also, does the now() result depend on whether daylight saving time is in use?
I understand now() returns the number of days since 0 Jan 0000, but does that mean "0 Jan 0000 UTC", or "0 Jan 0000 in the local time zone"?
Many thanks,
Neil
  2 Comments
Neil Rogers
Neil Rogers on 6 Dec 2019
So does that mean that if Alice in London types
now()
at exactly the same time as Bob types
now()
in Delhi, they should get exactly the same result, even though their local times differ by several hours?
I'm in the London/GMT time zone, so I can't easily test this without calling Bob.

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Accepted Answer

Steven Lord
Steven Lord on 6 Dec 2019
If you want control over the time zone, you probably want to use datetime instead of now.
N1 = datetime('now', 'TimeZone', 'local')
N2 = datetime('now', 'TimeZone', 'America/New_York')
N3 = datetime('now', 'TimeZone', 'Europe/London')
See this documentation page for more information about working with dates and times with time zones. If you want to know if a datetime occurs during Daylight Savings Time, use isdst. When called on datetime arrays without a time zone set, isdst return false.
  3 Comments
Neil Rogers
Neil Rogers on 9 Dec 2019
After setting my computer's timezone to non-GMT, I tested this method and it doesn't work as expected. datetime('now','TimeZone','Etc/UTC') still returns the local time(!).
My best solution now is to use the java method
utc = java.lang.System.currentTimeMillis / 8.64e7 + datenum(1970, 1, 1);
This returns a serial date number for the time in UT, and seems to be consistent across time zones - at least for my system; I'm not sure if it is true for all systems.
If anyone knows of a simple Matlab alternative to this, please let me know.

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More Answers (1)

Jeremy
Jeremy on 6 Dec 2019
From the documentation for
now
Limitations
  • MATLAB Online returns the current date and time in Coordinated Universal Time (UTC) rather than local time.
  1 Comment
Jeremy
Jeremy on 6 Dec 2019
Additionally, you can use the syntax
[c tf] = clock
Where tf will be a logical that is 1 (true) if the current local date and time occur during Daylight Saving Time in your current time zone, and 0 (false) otherwise.

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