Only edit selected Sheets in workbook

Re: "so if I'm making changes in August I don't need the last 7 months updated, just the remaining 4."

Run from "Developer" - "Macros" or if you have a macro button on every sheet:

Sub Another_Way()
Dim a As Long, sh As Worksheet
a = ActiveSheet.Index
    For Each sh In ThisWorkbook.Worksheets
        If sh.Index > a Then sh.Range("K12").Value = "Hello there!"    '<---- Change as required
    Next sh
End Sub

You can also have all the sheet names somewhere in your "Dates" sheet, select the cell that has the month from where you want to start updating and run adjusted above code.

Sub Another_Way_2()
Dim a As Long, sh As Worksheet
a = Sheets(Selection.Value).Index
    For Each sh In ThisWorkbook.Worksheets
        If sh.Index > a Then sh.Range("K12").Value = "Hello there!"    '<---- Change as required
    Next sh
End Sub