MenoData/Time4J is an open source project licensed under GNU Lesser General Public License v3.0 only which is an OSI approved license.
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ISO 8601 contains durations and time intervals which are totally undervalued! (https://en.wikipedia.org/wiki/ISO_8601#Time_intervals)
They are so convenient. Every tried to store a tuple of datetimes to model a time interval? E.g. a meeting that takes place on 2021-05-01T12:00:00Z and takes two hours. Don't store it as two datetimes! Store it as an interval: "2021-05-01T12:00:00Z/P2H"
Or are you creating an API where a duration or a time interval is expected? E.g. "give me all sales in this time period..."
Please use time intervals for that. For JVM developers, there is a library out there that has amazing support: https://github.com/MenoData/Time4J
For python developers, there is pendulum which supports most of the functionality.