I think you need some code for this. For example we will call your date field dteTrans.
In the subform when the first record is entered we will update dteTrans to the current date.
To do this we use the beforeupdate event of the first field in the subform, in which data entry will occurr.
Me.dteTrans=Date()
However, as this value is not saved it may dissappear as you move about the form.
Alternatively you could set dteTrans default value to Date()
HTH