I find a spring planting of bok choy or other Asian brassicas usually bolt unless I start the plants extremely early indoors, say late March, and get them into the garden before late April at the latest. Waiting longer I think just pretty much ensures bolting because the days are getting very long and temps are rising.

For fall planting, I start seeds generally around July 4th to mid-late July, transplant in August, and generally have only a few bolt out of the lot.

In my area I direct sow starting in the 3rd-4th week of July. I can sow later, say in early sept, for baby Boc choy.

I agree with denninmi, I planted early March and has bolted already. The lengthening days and additional day heat - along with cold night temps - causes the bok choy and other asian greens to bolt.

Continue to plant through the season in good soil and keep moist.