That's very common. They have the stamp marked for 6 months from today's date and they stamp all passports the same, unless exceptions.
They ask you for long are you going to stay in the US to make sure you're not going to work and sounds reasonable for the reason you're going. If everything is OK, they stamp with 6 months.
I'm sure you can stay there until Sept 26 at midnight.
One exception when they do not mark the 6 months is for example (this happened to me), when you go out of the US and come back in. For example:
- 3 months in the US
- Leave for one week to Canada/Mexico
- Return to the US
In my case, I stayed for 5 months in total. 3 months before and 2 months after a short trip to Mexico.
Because many people leave the country for a few hours/days to extend their passport valid period (ie, 6 months + 6 months), when you return they might not want to extend your stay for another 6 months. In my case they stamped it with the initial 'Valid Until' they put the first time I went in.
Photo of my passport with 2 stamps and same date: