Aesculus × carnea
It is a hybrid of the American Red Buckeye (Aesculus pavia) and the European horse-chestnut (A. hippocastanum). According to the Missouri Botanical Garden it was discovered in 1812. There's a story there.
The Red-Flowering Horsechestnut was introduced to California very early on. John Rock's Nursery in San Jose sold it in 1888. The California Nursery Company in Niles had it in the 1893 catalog..
Three baby chestnut seedlings this year. I am assured by an unknown (to me) expert on Facebook that the seedlings should look like the parent. Has one of them crossed with the California Buckeye (Aesculus californica) in the parking lot at the park? I will check if the bloom period is the same. Right now the buckeye has leaves again. I've seen flowers on other buckeyes (3/8/2016), but haven't noticed on ours. Will report back! If blooming periods do not overlap, then we can expect a red-flowering horsechestnut.