|If it is left justified in the cell and does not change into a number when formatted as General, then it is seen by Excel as TEXT, not as a DATE.|
So what you have is a Text string that looks like: 2010-12
To convert this Text string into a usable date you can do something like:
With your Text string of 2010-12 in Cell A1
in Cell B1, first format as Date: YYYY/MM/DD
then enter the formula:
Since you only have the Month & Year, the above formula defaults to
the first Day of the month.
So you get a Real Date of 2010/12/01, the first day of the month.