Time zone Time difference
 

I have a specific time in cell A1, I have an option of entering either EST,
CST, MST and PST in cell B1.
I have a specific time in cell A2, I again have an option of entering either
EST, CST, MST and PST in cell B2.

I want to calculate the actual time difference taking the timezone into
consideration and put it in cell A3.

Actually I wanted to do this in javascript because I was looking for an html
file. If we do this in excel and save it as webpage, will it work???