Example of the behavior I'm seeing:
I cast SW:P with Berserking and my SW:P starts ticking on the target every 1.62 seconds for 17.82 secs.
A Shadow apparition spawns, and hits the target.
Even if berserking is still active... the SA will add 1.62 seconds to the duration of the DoT, however, the tick period is then reset to my non-berserked haste, 2.26 seconds.
This effect occurs with both haste rating buffs (Volatile Talisman of the Shado-Pan Assault), and % based buffs (Berserking, PI, Legendary Meta).