Calculating Differences in Dates & Times
with Excel
I recently
rewrote this Q&A so I could submit it to be published without
screen captures. The original Q&A is below, with the images intact.
The rewrite clarifies some of the concepts, and is worth a read.
Enjoy. 

There are several different techniques for calculating the
difference between two dates or two times in Microsoft Excel. There
are several functions you can use to accomplish this task, however
this simple lesserknown technique works equally well in most
situations, and is easier to use.
In Excel, dates are stored as a number. Technically, the value of a
date is stored internally as the number of days since 12/31/1899. To
test this, just enter the number 1 in any cell, then rightclick on
that cell and select Format Cells. Change the format to Date and
click OK. Now you'll see the date 1/1/1900. That's one day since
12/31/1899.
Conveniently, time values are stored internally as fractions of a
day. So if you want to store the time 12:00 AM, that's simply a 0.
If you want 12:00 PM, that's given a value of 0.5, or half a day.
The other time values are calculated accordingly. 6:00 PM is equal
to 0.75.
Knowing this, you can easily calculate the difference in days
between two dates by simply subtracting them and making sure that
number is formatted as a date. For example, in cell A1, enter the
date 3/1/2012. In cell A2, enter the date 3/4/2012. Now in cell A3
you can subtract them:
=A2A1
The resulting value should be 3. There are three days between those
two dates. Excel automatically formats this as a number for you.
Calculating time values, however, gets a little trickier. Put the
time value 5:00 PM in cell B1. Put 6:00 PM in cell B2. Now subtract
them:
=B2B1
Excel returns the value of 1:00 AM as the result, which is not quite
what you want. That's OK. Just format this cell as a Number type
(again, rightclick and select Format Cells, choose Number, click
OK). Now you see the value is 0.04. Remember, however, that Excel
treats time and date values as whole days. So what you have now is
0.04 days. Let's multiply that value by 24 to convert it to hours.
Change cell B2 to read:
=(B2B1)*24
And now you can see the result is 1.0, which is what we expect.
There is a 1 hour difference between 5 PM and 6 PM. This technique
also works if you have a value that is a mixed date/time. For
example, if you change B1 to "3/1/2012 6:00 PM" and change B2 to
"3/2/2012 7:00 PM" you will see that there are 25 hours between
those two times.
Once you get used to it, you'll find that this is a much easier
method for calculating the differences between dates and times than
using all of the various functions that Excel provides.


Q: I'm using Microsoft
Excel. How can I take two columns of times and calculate
the total difference in time between them. For example, column A is
the time an employee clocked in and column B is the time the clocked
out. I want to know how many hours each employee worked.

A: This is simply a
matter of subtracting the two times, and then formatting the cell as
a number. Take this sheet for example:
If you want to put the total hours
worked in column C, all you have to do is subtract A from B:
But now you're left with a time.
That's because Excel assumes you're still dealing with times. Now,
just rightclick on the cell and select Format Cells. Change the
format of this cell to a NUMBER.
Now you get 0.08 which is fine...
keeping in mind that Excel is thinking in DAYS, not hours. Just
multiply this result by 24, and you have your hours worked.
Autofill down the column...
And you can see all of the hours
worked. Of course, now you have a problem for your overnight
workers. If they start at 11pm and finish at 7am, this won't work.
But that will be the topic of a different Tip, or watch my full
class on Excel Dates & Times: I cover dates and time in much more
detail in my
Excel 232
tutorial. 
By Richard Rost
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