|Here's what you need to consider:|
If you want to automatically add a value to a cell on a specific date, you have to open the spreadsheet on that date.
If you want to automatically add a value to a cell on or after a specific date, you can open the spreadsheet anytime you want, but you have to have a way to prevent the value from being added more than once. That means you have to have some sort of "flag" that tell's Excel that the value has already been added.
This example, when placed in the ThisWorkbook module of the VBA editor, will compare the current month against the value in A1 whenever the workbook is opened. If it's greater than that number, it'll add 400 to A2 and set the value in A1 to the current month.
You'll need to reset the flag in January, but that's only once a year.
Private Sub Workbook_Open()
'Compare today's month against value in A1
If Month(Now) > Sheets(1).Range("A1") Then
'If it's a new month, then add value to A2 and increment month flag
Sheets(1).Range("A2") = Sheets(1).Range("A2") + 400
Sheets(1).Range("A1") = Month(Now)
Note: This example is just to show you that something like what you want can be done. If you need something different, you'll need to be very specific with your requirements.