Todd Wolfson

Software Engineer

April 22, 2016

Until recently I always thought I had a decent grasp of timezones. It turns out my previous assumptions were reasonable but not practical. I thought:

  • Saving/using a numerical timezone offset (e.g. -0600) was "good enough"
  • JavaScript wasn't great at dates but it was better than most since it had the Date primitive

As it turns out: nope and nope.

Numerical offsets vs timezones

Timezones are typically based on geographical locations. For example, we have the IANA timezone America/Chicago which can represent Central Time for the United States.

Depending on the time of year, this will be a numerical offset of -0500 or -0600 from UTC. The change depends on the United States definition of daylight saving time. For example:

2016-01-01T00:00:00Z + America/Chicago -> 2016-01-01T00:00:00-0600
2016-04-01T00:00:00Z + America/Chicago -> 2016-04-01T00:00:00-0500

Unfortunately, regions can share the same offset (e.g. another country North/South) so if we save/restore the data, then we won't know what country the original datetime corresponded to.

2016-04-01T00:00:00Z + America/Chicago     -> 2016-04-01T00:00:00-0500
2016-04-01T00:00:00Z + America/Mexico_City -> 2016-04-01T00:00:00-0500

In most scenarios, this can be slipped under the rug but if you want to let users create/edit timezone-aware datetimes, then it won't work.

Additionally, in PostgreSQL all datetime values are stored in UTC with no preservation of IANA timezone. As a result, we must save both the UTC value and the IANA timezone in 2 separate columns.

JavaScript frustrations

In JavaScript's Date implementation, there is support for numerical offsets but not for IANA timezones. As a result, we need to perform some guess work. Thankfully there are a few solutions:

Once we have the timezone, we recommend to stick to using moment with moment-timezone to prevent any loss of IANA timezone. This includes when saving/restoring from a database.

Further reading

Here are some resources I found practical when reading up on timezones:

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