adding 1 to a time (cross mid night)

[lezawang]

Hi
I was watching a video on how to subtract 1:00 am from 11:00 pm (cross midnight). So the author added 1 to the am time and then subtract it from 11:00 pm.

Before adding 1 to 1:00 am, if I click on that cell I see in the formula bar

1:00:00 AM

when I add 1, see

01/01/1900 1:00:00 AM

My questions

1) What is the idea behind adding 1?
2) Why after adding 1, the formula bar showing 01/01/1900 1:00:00 AM (date and time)? what does 1/1/1900 means?

Thank you very much. The link to the video is below:

https://www.youtube.com/watch?v=pEamL2vgXus&list=PLsYjn_fAfG42S1JmTFr7B5eTQRvm5w3sF&index=126

 

[Yongle]

Time is a decimal less than 1
Dates are integers
Adding 1 to a time is adding one day to it - which is what happens when midnight is crossed

Try this..
In A1 enter formula
=NOW()
In A2
=A1+1
In A3
=A1+1.5

 

[lezawang]

Thank you very much for the help.

 

[jasonb75]

Adding 1 is an outdated method, somebody discovered that =MOD(B1-A1,1) works just as well.
(A1 contains start time, B1 contains end time).