SlashIn ( /in )
Provides the /in command for delayed execution. Also provides /slashin, in case of conflicts with other addons providing /in.
Many addons have provided this feature
-- so many that some users assume it's a built-in command. Long, long ago Cosmos provided it. In the Ace2 era, almost every Ace2 addon provided it.
Say "hi" in 1.5 seconds:
/in 1.5 /say hi
Use your Argent Crusader's Tabard to teleport, then put your other tabard back afterwards:
/equip Argent Crusader's Tabard
/use Argent Crusader's Tabard
/in 12 /equip Scryers Tabard
Many types of commands can't be delayed, especially ones that cast spells. If you receive an error dialog saying "SlashIn has been blocked from an action only available to the Blizzard UI," then that command can't be delayed.
The timer library can only generate events up to 10 times per second. Times with higher precision (more than one decimal place) are supported, but for example /in 2.0 and /in 2.01 aren't actually going to happen 0.01 seconds apart. They'll either happen together, or 0.1 seconds apart. (InDemand has the same limitation, but doesn't document it.) In practice this limitation really doesn't matter.