Auto Fill date based on the first item?

Hi I've been tasked with making a fillable excel form for employees of a small business to fill out their own timesheets. We used to use a pre-made one but it's no longer fit for us.

In the premade one, it didn't provide any functions for me to work backwards from, so I'm asking here.

Something the form could do is, by entering one date in one cell, the rest of the month would automatically fill itself out. (IE if you filled in that the first week began on january 01st, the rest of the form's mondays, tuesdays, etcs would also have the correct date attatched)

Any advice on how to automate this?