You'd use the precondition
t, with a min and max time. This and many other conditions for triggering events, are documented on the
event preconditions section of the event data page on the wiki :) I also really, really recommend
unpacking the game files so you can start poking around in the vanilla content and see how different things are put together. Events (at least, the ones that aren't hardcoded somewhere) are in Data/Events, and they're collected in files for the locations they take place in. (e.g. Shane's forest events are in Data/Events/Forest)
For adding NPCs after some condition -- I'm assuming you're using Content Patcher -- you can look at CP's many many conditions for all your patches on its
tokens and conditions guide. You can use the year token to only apply your patches on or after a certain year.
This section has an example for applying a patch only after y1, for example!
CP is so extensively documented and has so many features that it can be really overwhelming if you're unfamiliar with it and try to read it straight through. I think it's simplest as a reference when you need to find something specific :) (Remember there are also tons of custom NPCs modders have made that you can look at for examples, some of them quite fancy.)