Date functions

!!! decade means 10 years!!!

if you can do with the last day of April, then this will work for you:

=EOMONTH(A1,-1)

if you want a date that is somewhere between 20th and 30th (excluded), then you can go ahead and deduct 7 days (just an example) from the above:

=EOMONTH(A1,-1) - 7

remember to "date" format the cell in which this formula is placed.