That's not a date

Have you ever tried to enter a date like:

and gotten 35976.00, a value instead of a date?
You may be the victim of the indavertent change gremlin. It sounds like the cell in question been set to a Number format. When a cell's format is set to the default (General), entering a date will cause a standard date display. If the cell is set to a Number format, entering a date will result in Excel displaying the number format of that date.
To get back to normal, click the cell and choose Format--Cells. When the Format Cells dialog box opens, click the Number tab. Select General and click OK. This will have no effect on a previously entered date. You'll have to re-enter the date in the cell.