Google sheets date replace help

[Joel1122331]

Can anyone please help im trying to make a sheet that will update the next due date on a payment
eg if weekly then if A1 = today (=Today()) , B1 = start date (date of first payment manually entered) , C1 = weekly increment =(B1 + 7) and
D1= next payment due (C1+7)

but i need a app of formula that if C1 = A1 then replace B1 with today

i hope this makes sense thanks in advance for your help