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Define a named range "dates" to be =OFFSET($A$1,1,0,COUNT($A:$A))
Define a named range "values" to be =OFFSET(dates,0,1)
Fill D2:D13 with 1/1 to 1/12, formatted as MMM (so will appear as Jan-Dec)
In E2 enter =MAX((MONTH(D2)=MONTH(dates))*values) as an array (ie with CTRL-SH-enter instead of just enter)
Fill this down to E13
Continue to add data to columns A:B for the rest of the year