Dates that you enter into an Excel workbook have numerical significance. Excel converts the date into a number that reflects the number of days that have elapsed since January 1, 1900. Even though you won't see this number (Excel displays your entry as a normal date), the number is used whenever you use this date in a calculation. Times are also considered values. Excel sees them as the number of seconds that have passed since 12 a.m.
Follow these steps to enter a date or time:
After you enter your date or time, you can format the cells to display the date or time exactly as you want it to appear, such as September 16, 2003, or 16:50 (international time). If you're entering a column of dates or times, you can format the entire column in one easy step. To format a column, click the column header to select the column. Then open the Format menu and select Cells . On the Numbers tab, select the date or time format you want to use (you learn more about formatting text and numbers in Lesson 9).