12/30/1899 is the displayed value for a date field containing the value 0. Remember, date/time fields are double precision numbers, NOT strings. 12/30/1899 is the origin date. 1 = 12/31/1899 and -1 = 12/29/1899, etc. The decimal portion of the number contains the time.
If each subform record needs to contain the same value obtained from an unbound field on the main form, there is something wrong with your design. Add a new field to the main form's base table and store the value there, ONCE, instead of with EACH subform record.
When referencing form fields from WITHIN a form's class module, it is best to use the Me. syntax as it is more efficent, shorter, and has the advantage of intellisense. Only use the formal Forms! syntax when you are referencing a form from outside of its class module. To reference controls on the main form from a subform, use - Me.Parent.fieldname