When you need to work with time values, you simply extend the Excel date serial number system to include decimals. In other words, Excel works with times by using fractional days. For example, the date serial number for June 1, 2007, is 39234. Noon (halfway through the day) is represented internally as 39234.5.

The serial number equivalent of one minute is approximately 0.00069444. The formula that follows calculates this number by multiplying 24 hours by 60 minutes, and dividing the result into 1. The denominator consists of the number of minutes in a day (1,440).

=1/(24*60)

Similarly, the serial number equivalent of one second is approximately 0.00001157, obtained by the following formula: 1 divided by 24 hours times 60 minutes times 60 seconds. In this case, the denominator represents
the number of seconds in a day (86,400).

=1/(24*60*60)

In Excel, the smallest unit of time is one one-thousandth of a second. The time serial number shown here represents 23:59:59.999 (or one one-thousandth of a second before midnight):

0.99999999

Table 16-2 shows various times of day along with each associated time serial number.

TABLE 16-2
Times of Day and Their Corresponding Serial Number

12:00:00 AM (midnight) 0.00000000
1:30:00 AM 0.06250000
3:00:00 AM 0.12500000
4:30:00 AM 0.18750000
6:00:00 AM 0.25000000
7:30:00 AM 0.31250000
9:00:00 AM 0.37500000
10:30:00 AM 0.43750000
12:00:00 PM (noon) 0.50000000
1:30:00 PM 0.56250000
3:00:00 PM 0.62500000
4:30:00 PM 0.68750000
6:00:00 PM 0.75000000
7:30:00 PM 0.81250000
9:00:00 PM 0.87500000
10:30:00 PM 0.93750000

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