Implementing duration or time into if an if statement

I'm after a bit of help regarding if statements and time.

So basically I need some way of being able to tell me when an indicator has changed.

For example excel gives me a bunch of signals such as on, off and hold. I need a function to tell me when the signal changed to on. SO if the signal changed to on at 1240pm and i look at my computer at 1pm I need some way of being told how long that indicator has been on for or what time it changed to on.

I was thinking of something a long the line =If(cell nu=on, current time, no)

But current time obviously would update so i almost need to it freeze if the statement is true.