Say you have a spread sheet where you have have entered weekly data. *column
a contains the date the week ended and column b contains the weekly results
column c is the MTD column that adds previous weeks and column d is the YTD... *
How do I have a MTD column ...

To fill in column C, try putting
in C1: =B1
in C2: =IF(A2="","",IF(MONTH(A2)=MONTH(A1),C1+B2,B2))
then copy C2 downward as far as needed.

This assumes that results for a week are counted toward the month that
contains the last day of the week. Otherwise, there doesn't seem to be
enough information in the spreadsheet to calculate the desired result
-- we'd need daily data.