Unix command line with GNU date:
date --iso-8601=seconds
date --iso-8601=seconds -u # UTC
date --iso-8601=seconds -d @1234567890 # specified time stamp
date -d 2017-06-29T12:28:57-07:00 +%s # other direction
Python: https://stackoverflow.com/questions/2150739/iso-time-iso-860...
Using a network API for this kind of conversion function that's already built into programming languages reminds me of the left-pad controversy. You don't need to outsource basic programming tasks to third-party libraries, and you also don't need to outsource them to third-party network services.