[theunamedguy]

It's just a conversion factor to make the units come out nicely. He could have easily done T = 1/f to give the refresh interval in seconds, but with such a short duration, it helps to convert it to nanoseconds (1e9 ns = 1 s).

[throwaway77790]

Oh, I see now. Thanks a lot. Makes sense. 1bil ns/1s