I suggest you (a) save a copy of your file as a clone (no records), (b) fix existing data to conform to your preferred format, and (c) import it into the clone.
A calculation =
GetAsTimestamp ( GetAsNumber ( YourOldTimestampData ) )
has a good chance of converting the old data to the current format - provided the file isn't messed up to a point where the old timestamps aren't recognized anymore as timestamp at all. Also watch out for day and month being interchanged.
Another way to normalize your data to a common standard is to format the timestamp field on a layout as m/d/y h/m/s, then export as text while selecting the 'Apply current layout's data formatting…' option.