Automatically Advancing by a Month

Jim has a need to advance the date in a particular cell by one month at midnight on the 14/15 of each month (00:00:00 on the 15th), and he wondered how it could be done.

As with many problems in Excel, the answer depends on the nature of the data involved and exactly what you want to do. If the date in the cell is today's date, and you simply want to have the cell display the current month up through the 14th, and then next month after that, then you can use a formula such as the following:

=CHOOSE(IF(DAY(NOW())>14,MONTH(NOW())+1,MONTH(NOW())),
"January","February","March","April","May","June",
"July","August","September","October","November",
"December","January")

This formula returns the name of a month, not a date. If you prefer to have a date returned, you can use this formula:

=IF(DAY(NOW())>14,DATEVALUE(IF(MONTH(NOW())=12,1,
MONTH(NOW())+1) & "/" & DAY(NOW()) & "/" & IF(MONTH(
NOW())=12,YEAR(NOW())+1,YEAR(NOW()))),NOW())

Both of these formulas account for the "end of year wrap-around" when you advance from December to January. A shorter version of this last formula can be created if you use the DATE function instead of the DATEVALUE function:

=DATE(YEAR(NOW()),MONTH(NOW())+((DAY(NOW())>14)*1),1)

This formula, unlike the DATEVALUE example, always returns a date that is the first day of any given month.

If you really want to advance the value of a particular date in a cell, then you must use a macro to do the task. Further, you must make sure that the macro only runs once a month, at a particular time on a particular day. For instance, if you wanted the macro to run at 00:00:00 on the 15th of each month, you would need to set up the macro so that it checked the date and time, and then ran at that particular date and time. You would also need to make sure that the workbook containing the macro was open over that date and time.

The following macro will fetch the date from a cell and increase it by a month. The macro assumes that you have a named range, DateCell, which refers to the cell to be updated.

Sub IncreaseMonth()
    Dim dDate As Date
    dDate = Range("DateCell").Value
    Range("DateCell").Value = _
      DateSerial(Year(dDate), _
      Month(dDate) + 1, Day(dDate))
End Sub

To make sure that the macro runs at the appropriate time, you would need another macro. The following macro is designed to be run whenever the workbook is opened:

Private Sub Workbook_Open()
If Day(Now) = 14 Then
    Application.OnTime ("23:59:59"), "IncreaseMonth"
End If
End Sub

Notice that this particular macro sets the OnTime method so that it runs the IncreaseMonth macro at 23:59:59 on the 14th. This date and time was chosen because it is easier to catch than is 00:00:00 on the 15th.

Remember that the IncreaseMonth macro will only run if you open the workbook on the 14th, and then leave the workbook open until the 15th.