I'm not sure if I'm missing something, but why can't AST use time dilation on themselves?

I understand mostly in a party situation you may want to extend buffs on a party member instead of yourself, but this makes time dilation in solo play completely useless and even in party play you may want to extend buffs for yourself anyways.

Again, I am not sure if it's just me who is doing something wrong and can't use time dilation on myself or not, but I really don't understand the reasoning behind restricting time dilation to party members only if this is the case.