Excel Time and Elapsed time formula

Hello,
I have need of some help. Thank you to anyone who replies or takes the time to read this. I am new in my Administration position and the biggest time suck is literally addition. I am not a lazy person however, I would like more time to make changes to the processes the previous admins in my position did not change for efficiency purposes.

I would like to enter time into my excel database in basic number form using a 24 hour format into two cells. One for beginning and one for ending. I have been trying for a few hours to fix the end result. I cannot get excel to calculate the elapsed time if it exceeds a 24 hour period, or from evening to morning.

for example: 1400 = A1, 0700 = A2. I would like A3 to calculate elapsed hours.

If I need to put in the date, I can do this. Is there a way to enter the date in number format without the hashes, and excel read the date and time so it can calculate the elapsed time in hours?

Thank you for your time and assistance.

Thank you for the formula! I cannot get the formula to work from one date to another though, do you have a way to write it so it will calculate across multiple days?

To clarify further:
Currently I am using custom format of my cell as this, "00\:00 00\\00\\00." I type in 1645062513 to get out 16:45 06/25/13. I would like to take two cells that read something like the below two options, and calculate the elapsed time period in hours.
A1= 1645062513, which reads: 16:45 06/25/13
A2= 0745062613, which reads: 07:45 06/26/13

Is there a way I can take the elapsed time from these two dates? I do not mind inputting my data differently however, it would be nice to streamline the input to just numbers. Thank you all again for your time and assistance.