Time Difference between two date format

Hello,

Whenever i try want to calculate the time difference between a [Date Time] on/before 12th of every month to [Date Time] on/after 13th of every month gives error value. The actual issue is of the Date format.
For Example :

Cell A1 = 07-12-2014 12:12:08 -------> 12th July 2014
Cell B1 = 07/13/2014 13:04:13 --------> 13th July 2014
Cell C1 = B1 - A1 ---------------------> gives "#Value!"

Require output in HH:MM:SS format.
I have to replace manually the date format from either "-" to "/" or "/" to "-"

Kindly help..

Dear oeldere,

As requested....

Dear oeldere,

"format as value" --

Dear oeldere,

You have manually changed the format of the date in Cell A2 and Cell B2. In such cases i will have change the dates in all the cells to same format to take the time difference.(Find and Replace--- Ctrl+k) to do the changes...
In the first post also i have mentioned "The actual issue is of the Date format."
Any easier method..

the reports that i get has different formats of dates...
The format of dates changes after 12th for every month...
On / Before 12th the format is MM-DD-YYYY and
On / After 13th the format changes to M/DD/YYYY.

It is not possible for me change the format at multiple places of the my multiple servers.
The dates before 12th coincedes with month's of the year. check out using filters on the excels provided.
The format is surely in mdy on both days..

So what changes in settings are needed in my regional world?
Any solution to the 12days of phobias I face every month. As they are the minor part compared to rest of the month.
Thanks in advance.

You can write the formula in this way = Time(B1-C1)