So just so I understand better James. If I accept your premise that most (common varieties of non-dwarf) pumpkins are ready in 120 days, 120 days from May 5 is Sept 5. OK, I get it that you can't count back from Halloween, if your first frost date is before that. My first frost date is Oct 15; Indiana isn't very far from me and the same latitude. If their first frost date is the same, why wouldn't they plant a month later, June 5 not May 5? And they always say don't plant squash/ pumpkins until the soil is well warmed up. May 5 seems a bit early for that.
You may be right. since your frost date is a month later than mine, you could likely plant June 5 and get good pumpkins by frost. Here I plant pumpkins May 5 and they are just getting ripe about mid Sept when we get a frost. You see this is still six weeks until Halloween, and somewhere around 130 days from planting. It takes a little longer than the package says at this altitude.
If you plant any squash right now with 110 day or less completion time, you should be good for harvest at Halloween. That is supposing you don't get frost before October 31. A frost will stop the development of fruit.
You may do better right now to plant some 95-100 day pumpkins if you can find them.
rainbowgardener, do you grow pumpkins, and if so when do you plant them? Also do you find that they finish in the time the package says? Here at this altitude of 5000 ft, everything takes a few days longer than advertised. This is something we can only learn after several seasons of growing varieties. Each year is different too. We had an early warm up this season and my corn is much higher that usual at this time. Gardening is always a risk, and a new adventure each season. That is part of the fun.
Gardening at 5000 feet elevation, zone 4/5 Northern Utah, Frost free from May 25 to September 8 +/-