Increasing Value In Spreadsheet By Set Amount Monthly?

I need a series of cells to increase by varying amounts after each month, however this needs to happen automatically so if I send the spreadsheet to someone else, it will update for them automatically. I'm hoping that I can set up a function that will ONLY increase the cells after 4 weeks after they select a date from ANOTHER drop down menu (i.e they select the starting date from cell A1 - then input their starting numbers into other cells which then increase on that same date, 4 weeks later).

The cells that need to increase already have a formula in them as well; as this number is their predicted 1RM based off formulas in other cells.

I will attach a screen shot when able.