I find that UA is actually pretty good on time estimates (although they do tend to the optimistic on turn times). UX operations on the other hand tend to be pretty bad. It seems that they don't update the UA computers particularly regularly or accurately (at least SkyWest at PDX). It is pretty common to have the info in the RCC be "on time" but then get to the gate to find the UX GA tell you significant delay. The RCC folks at PDX generally are happy to call the gate to get a real scoop since they seem to know about how iffy UX data is. As to the creeping delay issues - take a look at the inbound and you can see the issues. Often you will see the inbound creep as ATC spins folks longer than anticipated. The inbound times tend to track pretty accurately and the outbounds tend to be arrival time + turn time where the turn time is a function of the aircraft type (they amazingly have been using 40 minutes for 57s of late which is a lot more realistic that 20 minutes of old).