My experience on an eastbound TA was that you got exactly 24 hours of access. So if I purchased it at 10am, and the clocks went forward an hour at 12 noon, it ran until 11am the following morning.
I would expect the reverse to happen on a westbound TA, e.g if I purchased at 10am, and the clocks went back overnight, the package would run until 9am the following morning.