Upvote:6
If your visa is valid until a date, that includes that date.
Don't worry, this is an extremely common question, common enough that there's a whole page on it on the US travel site.
However, remember, as pointed out there - that your visa validity isn't necessarily how long you're authorised to stay in the US - the CBP at port of entry could determine that despite having a visa that expires in 3 months that you only need two weeks, and will stamp as such. Example, I flew into Boston for 2 weeks of travel - instead of being given the 90 days that NZers can get visa free, he just gave me enough time to complete my trip, and stamped accordingly.