Time: Convert 1 min to 15 min interval

Help with how to FIND the 1st one- the first 1 minute interval data point? This is where I was having trouble on my own. I didn't know how to subtract time. I was trying to look for the delta between the timestamps, and if it was less than 15 minutes (or something bigger than 1 min), then identify it.

If the data were ALL 1 minute intervals, this would be easier for me. It's tricky for me (not an expert) because it's mixed.

Is there a reason why your data goes backwards? Do you need to keep it that way?

Pete