Using a macro, you could do this *if* the name of the field (better, an
invisible textbox that is bound to the field) is the same in all the forms.
Macros allow you to drop off the form name if the macro is being called from
a form and you want to use a control or field that is in that form.

MacroName: macSetTodayDate

   Condition:  [NameOfCheckBoxControl] = True
   Action:  SetValue
       Item:  [NameOfTheDateControl]
           Expression:  Date()

   Condition:  [NameOfCheckBoxControl] = False
   Action:  SetValue
       Item:  [NameOfTheDateControl]
           Expression:  Null

The above macro should be run from the After Update property (when the
AfterUpdate event occurs) of the checkbox control on the form. Note that
each form must use the same name for the checkbox control too. This macro
will put today's date in the textbox if the checkbox is checked, and will
put Null in the textbox if the checkbox is unchecked.

A shorter way to do this same macro would be this:

MacroName: macSetTodayDate

   Action:  SetValue
       Item:  [NameOfTheDateControl]
           Expression:  IIf([NameOfCheckBoxControl], Date(), Null)