controlling cell entries

[mwvirk]

how can i stop user to enter values in 1 cell before filling some other cells.
e.g. i don't want user to give end-time in B cell before giving start-time in A cell because it is disturbing my sheet when it calculate time difference in C cell.
i don't mind is problem can be solved by conditional formatting or VBA code.
thanks.

 

[mwvirk]

just to add more:
i have sheet where i got lot of entries manually and some of them are getting filled up automatically based on other cells and also some cells are getting updated using VBA code.
in this situation, how can i use conditional formatting or vba code?

 

[shg]

You could use data validation (http://www.contextures.com/xldataval01.html) to not allow data entry unless some other cell is filled, but you should fix your time difference formula so it doesn't matter.

 

[mwvirk]

thank you.
i never used this before. can you please help me.