I can think of 2 ways to go with this, depending on how you want the first of
the month treated:
=EOMONTH($A$1-1,3)+1
OR
=EOMONTH($A$1,3)+1

Note: Per Excel Help:If this function is not available, and returns the
#NAME? error, install and load the Analysis ToolPak add-in.


Does that help?

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Regards,
Ron