if you want a Distinct Count for all records in the report then use a Ruuning total
CAll it something like "ColADistinctCount"
Field to Summarize=Column A
Type= Distinct Count
Evaulate- USe a formula 'datediff("M",date1,date2)<12'
Reset as: Never
Place this Running Total Field in your Report Footer to get your number.
Running Totals only work in Details or Footers and it muse be placed in a section that can evaulate everything it needs to.
Also, you can make a formula field with your date diff and temporarily drop it in your detail row to make sure it is giving you the correct data. If you invert the dates you will get a negative number and all of your records would then be <12 and give you some false positives.
Create a Formula field as "MonthTest" and insert this formula:
'datediff("M",table.date1,table.date2)'
Place it on your details row and see if you are getting the correct month difference. If it is giving you all negative values you need to switch the order of the date fields
'datediff("M",table.date2,table.date1)'.
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