If you experience problems with date formats not “sticking,” here’s a sanity-saving technique to make your dates appear in the same format.
Let’s say you had one date showing up as 6/1/2011 and the same date later in the column as 1-June-2011. The secret is Clear Formats. The Clear button is located in the Editing group (on the far right) of the Home tab. Click it and choose Clear Formats. The dates will initially appear as serial number dates, but they will appear in whatever date format you choose in the Number format field, in the Number group, on the Home tab.