A time kill is when a team kills an enemy immediately either before or after they would have been automatically resurrected (i.e., at the even-number minute mark in Priest Annihilation or Guild Battles). Time kills are intentional, and serve two purposes.
Killing immediately before a two-minute mark (within 2 seconds of the mark, so the team doesn't get a chance to resurrect) will base a target. The target will be killed, then instantly resurrected in their base, which forces the target to spend time returning to the fight. If a healer or other essential member is "based," the team must hold out without that member for 30 seconds or longer (until the based team member returns to the fight), or simply fall back and lose ground. Basing is most useful in Guild Battles, where the maps are wide and pathways are long. It is still somewhat effective in Priest maps like Fetid River and Burial Mounds, although the Resurrection Shrine is much closer to the battle zone.
Killing immediately after a two-minute mark is tactic used when you've killed most of the opposing party already. Snares and body blocks are often used to keep a target alive and in range until the other team resurrects (at their base), at which point the target is killed. This serves a variety of purposes; the opposing team must now fight without the maximum number of people. They also must use a resurrection skill when they return to the corpse of their fallen ally; it costs them either valuable time or the use of a Resurrection Signet. Thus, a successful time kill can put a team at a distinct advantage.