Posssible bug?

Thank you for reporting.
Can you confirm if the issue persists if you update to the version available on github? https://github.com/carmoreira/date-time-picker-field On my tests it seems it works correctly, but might need to test more.

How do I update from github? (not a techie)

Found the well hidden “Download ZIP” so ignore my query.

Hmm, I now have two instances of the plugin listed. What should I do?

You should disable the older version and only keep the new version enabled.