2019-07-22 22:53:30 +00:00
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---
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layout: docs
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title: ISO8601 vs Millis
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2020-03-28 15:35:28 +00:00
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permalink: dev/adr/0004_iso8601vsEpoch
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2019-07-22 22:53:30 +00:00
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---
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# ISO8601 vs Millis as Date-Time transfer
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## Context and Problem Statement
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The question is whether the REST Api should return an ISO8601
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formatted string in UTC timezone, or the unix time (number of
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milliseconds since 1970-01-01).
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There is quite some controversy about it.
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- <https://stackoverflow.com/questions/47426786/epoch-or-iso8601-date-format>
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- <https://nbsoftsolutions.com/blog/designing-a-rest-api-unix-time-vs-iso-8601>
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In my opinion, the ISO8601 format (always UTC) is better. The reason
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is the better readability. But elm folks are on the other side:
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- <https://package.elm-lang.org/packages/elm/time/1.0.0#iso-8601>
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- <https://package.elm-lang.org/packages/rtfeldman/elm-iso8601-date-strings/latest/>
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One can convert from an ISO8601 date-time string in UTC time into the
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epoch millis and vice versa. So it is the same to me. There is no less
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information in a ISO8601 string than in the epoch millis.
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To avoid confusion, all date/time values should use the same encoding.
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## Decision Outcome
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I go with the epoch time. Every timestamp/date-time values is
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transfered as Unix timestamp.
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Reasons:
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- the Elm application needs to frequently calculate with these values
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to render the current waiting time etc. This is better if there are
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numbers without requiring to parse dates first
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- Since the UI is written with Elm, it's probably good to adopt their
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style
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