So what would happen if outdoor light schedule changes?

So I have a sativa dominant strain I'm growing I don't know what it is exactly I got the seed from a friend. However I started growing it outdoor sometime in late January and it's about 2 feet tall and starting to bud but over the last week it's been slow. I was wondering if that could be from longer days now it gets about 13 hours of light if I leave it outside

yeah going from 18 to 13 will cause it to flower, going 18 to 16 will sometimes even

it will ether keep flowering or reveg