Ok, so lets say the date is in D1, and that value is 1/14/2019.
If cell F1, you can add the number of days to the date in D1, like =D2+25
Now in cell G1, you can place a formula that says
=if(TODAY ()>=F2,"Bill it")
That will give you a twenty-five day billing period.
Viola! Now place that example into your model and have a go!
After you have all of that set up, come back and we will format the bill it cells to turn red! Oh what fun!