why do so many people that live in the same area,have different dates to apply pre ems? some say Feb some say March who is right?

Hit the one in the middle.

For me it's simple. I want to stay ahead of the game. Specially against grassy weeds like crab. Look at the 5yr soil temp avg for Dallas. Feb 24th 5 year avg has been 56.6 degrees soil temp. 4 days of soil temp of 55 and crabgrass germinates. Cold and even freezing temps do not affect the efficiency of pre's. So I would always get it out sooner than later.

You'll see the temps on this map fluctuate. If we get a warm spell in Feb which isn't uncommon weeds will germ quickly. Look how the soil temps jumped right after the blizzard last year. Had soil temps that jumped up to 63.

https://www.greencastonline.com/tools/soil-temperature

Early February is correct for north central Texas. Those that say March are wrong.

Easy Pre schedule for N Central Texas
Valentines Day Barricade
Memorial Day. Barricade
Labor Day Barricade + Gallery

This is great advice, The holiday schedule is actually a good schedule to go by for the avg home owner who kinda/somewhat cares about their turf. Both for pre and fert. With a couple added notes

Barricade or Stonewall as discussed many times is Prodiamine. Great grassy weed control with long soil residual. If you want to do the Holiday schedule divide the yearly max by 3 and apply on those holidays.

The downside to the Holiday schedule on Pre's is most will do the Labor and Valentines and forget the Memorial app. That's why I like two 6 month apps of Barricade/Stonewall.

And on pre's the prill mix is pretty small typically. So when their yard is 2500 sq feet on the front and they have to measure out 5lbs out of a 50lb bag and spread that small of amount. 1st. It's not that easy to spread but can be done with multiple passes on the lower settings. 2nd. They're like no way this small amount works as I just took a couple scoops out of the bag and have so much left over. Even know the bag says it will cover 13K+ or so.