[fiatjaf]

I don't know how it is supposed to work: https://screenshots.firefox.com/43Qs5xpvJ3Jm1oHi/date-format...

Why are there two different fields? I imagined only one would do the job.

[lotkowskim]

It's to remove ambiguity (not all), i.e. you select a date and then specify how you want it to look. For example, the date 12/05/2017 can be interpreted as either DD/MM/YYYY or MM/DD/YYYY but if you select a date first and then describe your desired format we can exclude one of the possibilities.

[0x62]

Perhaps consider removing the choice? For example, have only one input (the desired format) and above it some text saying "How would you format '28/03/2018 21:37'?".

This removes the confusion of two inputs, and provided you select an unambiguous baseline date (day & hour > 12) it should be more clear.