If I use my move and standard actions in a round, then activate my greatreach bracers which extend your reach for one round, by RAW 6 seconds, would they be active on my next turn? Could I then activate my heartseeking amulet and use it before the end of my turn? In other words does a one-round action taken at the end of your turn carry to almost the end of your next turn?
If you start a 1-round duration as the last action of your turn, does it continue through your next turn?
A round takes 6 seconds, but there are no rules for the timing of individual turns. Instead of seconds, turns are tracked using initiative—and initiative doesn’t change during a turn. So your entire turn takes place within one moment—from a timing standpoint, the beginning of your turn is the same as the end of your turn. When you activate your greatreach bracers during one turn has no bearing on when their 1-round duration expires.
As for when that 1-round duration expires, we have an exising Q&A on that: When do spells expire? That includes the following quote:
For almost all purposes, there is no relevance to the end of a round or the beginning of a round. A round can be a segment of game time starting with the first character to act and ending with the last, but it usually means a span of time from one round to the same initiative count in the next round. Effects that last a certain number of rounds end just before the same initiative count that they began on.
(System Reference Document → Actions in Combat → The Combat Round, emphasis mine)
So no matter when you activate your greatreach bracers during your turn, their effect expires when the initiative count returns to whatever it was when you activated—which is when your next turn begins.
Thus, your greatreach bracers will not be active, and will require another swift action to activate, preventing you from activating your heartseeking amulet—they are therefore mutually exclusive.¹
- Unless you are a 7th-level ruby knight vindicator from Tome of Battle, whose divine impetus ability allows them to expend uses of turn undead in order to get more swift actions during a turn. This ability is unique, and alone would be a good reason to consider the ruby knight vindicator to be one of the stronger prestige classes in the game.
\$\begingroup\$ Thanks K. So the difference between end of turn (h.amulet) and round (g.bracers) is the bracers would still be in effect for AoOs. \$\endgroup\$– NathtfmJul 25, 2021 at 20:45
\$\begingroup\$ @Nathtfm Correct. Big difference, in the case of reach. \$\endgroup\$– KRyanJul 25, 2021 at 20:56
\$\begingroup\$ Minor nitpick: there are things you can do to change your character's initiative count (eg., delaying). While that wouldn't affect this scenario, there are potential shenanigans available if "until the target's next turn" abilities are in play. \$\endgroup\$– minnmassJul 26, 2021 at 16:36
\$\begingroup\$ @minnmass Delaying is still an action, so I’d argue that no, there aren’t any shenanigans—your turn comes up, the effect ends, and then you delay. The effect doesn’t resume or continue. \$\endgroup\$– KRyanJul 26, 2021 at 16:40
\$\begingroup\$ @KRyan: perhaps using readying would have been better... The point, though, was that effects that "end just before the same initiative count that they began on" don't always perfectly line up with the beginning of a creature's turn. \$\endgroup\$– minnmassJul 26, 2021 at 22:47