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+++ title = "ISO8601 vs Millis as Date-Time transfer" weight = 50 +++
Context and Problem Statement
The question is whether the REST Api should return an ISO8601 formatted string in UTC timezone, or the unix time (number of milliseconds since 1970-01-01).
There is quite some controversy about it.
- https://stackoverflow.com/questions/47426786/epoch-or-iso8601-date-format
- https://nbsoftsolutions.com/blog/designing-a-rest-api-unix-time-vs-iso-8601
In my opinion, the ISO8601 format (always UTC) is better. The reason is the better readability. But elm folks are on the other side:
- https://package.elm-lang.org/packages/elm/time/1.0.0#iso-8601
- https://package.elm-lang.org/packages/rtfeldman/elm-iso8601-date-strings/latest/
One can convert from an ISO8601 date-time string in UTC time into the epoch millis and vice versa. So it is the same to me. There is no less information in a ISO8601 string than in the epoch millis.
To avoid confusion, all date/time values should use the same encoding.
Decision Outcome
I go with the epoch time. Every timestamp/date-time values is transfered as Unix timestamp.
Reasons:
- the Elm application needs to frequently calculate with these values to render the current waiting time etc. This is better if there are numbers without requiring to parse dates first
- Since the UI is written with Elm, it's probably good to adopt their style