Pushing Dates Into Last Month

Kathy needs a formula that will change any date within the current month to the last day of the previous month. She also notes that it would be helpful to have a VBA macro that could do this to the dates in a range of selected cells.

There are actually several different formulaic approaches you could use to derive the desired date. These two are the shortest and easiest to use:

=A1-DAY(A1)
=EOMONTH(A1,-1)

The first one works because "day 0" of the current month is always considered to be the last day of the previous month. Thus, if the date in cell A1 is 9/25/21 you end up subtracting 25 (the DAY value) from that day, giving a date of 9/0/21, or 8/31/21.

The second formula works because of the inclusion of the second parameter for the EOMONTH function. This parameter is used to indicate how many months in the past (negative) or future (positive) the end of the month should be calculated.

In either case, you may need to format the cell containing the formula as a date. If you don't, what you see may be the underlying serial number used by Excel to calculate dates.

If you want a macro that will actually convert dates in a selected range into the last day of the previous month, the following will do the trick.

Sub LastDayPreviousMonth()
    Dim rCell As Range

    For Each rCell In Selection
        If IsDate(rCell) And Not rCell.HasFormula Then
            rCell.Value = WorksheetFunction.EoMonth(rCell.Value, -1)
        End If
    Next rCell
End Sub

The macro steps through each cell in the selected range and checks to see if it contains a date and is not a formula. (In other words, if the date in a cell is the result of a formula, the formula is not changed.) The EOMONTH function is then used, as previously described, to calculate the desired date.

It should be mentioned that the macro approach will also convert textual dates. For instance, if a cell contains, as text, the date "6/15/21," then the macro will change it to 5/30/21. Once the adjusted date is stuffed back into the cell, it will show as a date/time serial number, not as text.

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