Hi Melvin & Welcome to the Forum,

Can you share what you are trying to achieve?

If you put 1/1/2016 in a cell, in terms of a number it is 42370.

In Excel, time is calculated from 1/1/1900 which is equal to 1.

There have been 42370 since 1/1/1900

If you have...
A1 = 1/1/2016
A2 = 10/1/2016
A3 =A2-A1

The result is 274

There are 274 days between 1/1/2016 to 10/1/2016
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Sorry, just realized I may have misread your post

Maybe you want =SUMIF(A2:A20,E1,B2:B20)

where

A2:A20 = dates
B2:B20 = numbers
E1 = a date to search for