The problem: Excel does not want to recognize dates as dates, even though through “Format cells – Number – Custom” you are explicitly trying to tell it these are dates by “mm/dd/yyyy”. As you know; when excel has recognized something as a date, it further stores this as a number – such as “41004” but displays as date according to format you specify. To add to confusion excel may convert only part of your dates such as 08/04/2009, but leave other e.g. 07/28/2009 unconverted.

Solution: steps 1 and then 2

1) Select the date column. Under Data choose button Text to Columns. On first screen leave radio button on “delimited” and click Next. Unclick any of the delimiter boxes (any boxes blank; no checkmarks) and click Next. Under column data format choose Date and select MDY in the adjacent combo box and click Finish. Now you got date values (i.e. Excel has recognised your values as Date data type), but the formatting is likely still the locale date, not the mm/dd/yyyy you want.

2) To get the desired US date format displayed properly you first need to select the column (if unselected) then under Cell Format – Number choose Date AND select Locale : English (US). This will give you format like “m/d/yy”. Then you can select Custom and there you can either type “mm/dd/yyyy” or choose this from the list of custom strings.