|You probably do not have Dates & Times, |
but simply TEXT strings that look like Dates & Times.
To make sure your Date are really DATES,
select a cell and right click your mouse
select Format Cell, Select the Number Tab
then select General.
If your Date cell does NOT display as a five digit number,
then you have a TEXT string not an Excel DATE.
That because "Excel stores dates and times as a number representing the number of days since 1900-Jan-0, plus a fractional portion of a 24 hour day: ddddd.tttttt . This is called a serial date, or serial date-time."