It seems you have used a time field, which displays the current time from the system clock and the field is not locked. It would be better not to use a time field but to insert the current time as text and then it can't happen. You could probably do this witha macro, but I am not sure how compatible the Mac is. The following would work on a PC, and may work on a Mac.
There will be no record of the intended time in the document, so there's not a lot that you can do about it.
Code:
Sub InsertTime()
Dim orng As Range
Set orng = Selection.Range
orng.Text = Format(Time, "HH:MM:SS |")
orng.Collapse 0
orng.Select
lbl_Exit:
Exit Sub
End Sub
See
http://www.gmayor.com/installing_macro.htm