I found using timeLastModified from macOS drive to ExFat, macOS has higher precision than ExFat[0], so a different number. My little program lists the files to update, but I get files to update still, after updating them, because of the differences with the disk formats.

[0] from time: 2020-Nov-06 13:29:30.5897005, to time: 2020-Nov-06 13:29:30.58
Potential copy("/Users/joelchristensen/jpro/dpro2/AbcKids/LettersO/d.wav","/Volumes/PNY480/jpro/dpro2/AbcKids/LettersO/d.wav");

You can compare like time1 > time2 + 100.msec, though posix specifies only second precision.