Sunscreen and timing question. There are no dumb questions when it comes to taking care of our skin right?

Beauty Tips For Body Care
So I understand that we are supposed to apply sunscreen at least 15-20 minutes before sun exposure but when does the two hour window start for reapplication? Does it start once I got out side or as soon as I put the lotion on? If I apply my morning sunscreen at 7am, does my protection expire at 9 even if I did not go out in the sun yet?
Girls Blog 2015
Submitted by elbowdoodle