Hi Anas,
If you have specified a 'DATE' field in your geodatabase datatset and use the ArcPad Data Manager Toolbar in ArcMap to check-out your ArcPad Project, the forms that are automatically generated for use in ArcPad will include a 'Date Picker' for the 'DATE' field. All the field user needs to do is click on the date in the calender to enter that information into the 'DATE' field. No scripting required!!
Hannah Ferrier
ArcPad Development Team