Get time value from textbox and perform calculations?

I'm trying to retrieve a time value from a textbox and perform calculations on it.

Use ca开发者_StackOverflow中文版se example: User inputted time (8:00 am) - (30 minutes)

I'm getting the value by the getElementById method, but I am unsure how to make it read in a proper time format and do the calculations.

Could someone point me in the right direction, please?