Button click to populate a field with the current time

URLRoot() & "db/" & Dbid() & "?a=API_EditRecord&apptoken=MYAPPTOKEN" 
& "&rid=" & [Record ID#] & "&_fid_999=now"

& "&rdr=" & URLEncode(URLRoot() & "db/" & Dbid() & "?a=dr&rid=" & [Record ID#]))

The syntax above will populate the field fid 999 with the current date and time when the button is pushed as opposed to the old "stale" value when the page first loaded.  The issue is that it is the current date/time and not the time of day.  So you will need to rework your app or if you have existing data, have a new fields going forward which make use of the date/time stamp  and use formulas to calculate the Time of Day from them.

Your Time Card punch in / punch out use case is EXACTLY what that syntax is meant to address.