Early morning or when Mr. Sun has gone out for the night already. In my opinion, that's the best time to go running in an open field.
Some people though are very brave running under the heat of the sun. Don't they care about the damage it's causing their skin? Hopefully they're wise enough to put on sunblock to avoid any skin damage from the sun's harsh heat.
My son usually swims at 330 in the afternoon. Right across the pool is a track oval. I've seen local celebrities, like Edu Manzano and Will Devaugn, hit that oval under the heat of the scourging sun. I guess they're trying to avoid the thousands of runners frequenting the place after sundown.
They should check if getting very sick, due to dehydration, is covered by their health plan though.