Any instance that both the server and client have agreed on are the same instance will have server side changes replicated to the client, but not the other way around (for security)
This is everything in Workspace, Players and ReplicatedStorage (and some other less used services)
In almost all cases the server doesn’t get updates from the client. The client will always see changes from the server though. This affects pretty much every object and every property with a couple exceptions. So in a sense it does work as a one way remote just using Roblox’s default replication. You certainly could use this for a timer. I would suggest using a remote event when passing data just because it becomes clear in the future that you intend communication between client and server from that object since a value doesn’t communicate that by itself, but at the end of the day either works.