OnlineBookClub.org

We use "since" when we're sure about the period of time, we're talking about exact time i.e., Since 1921, since yesterday, since this afternoon, since last month. And we use "for" when we know the time but not about exact time i.e., for 3 days ( which 3 days either the three days of this week, or month), for 2 hours ( when this morning, yesterday 3 hours, last week three hours)

Thsts also true. With since it is also used in synonym to est. Which means in existence since...

So whenever we are aware of the time and when it all started we use since.

Since is typically used to indicate the start of a period of time - example, since 1921, as you pointed out; whereas, for is used to indicate a period of time - for three days, for a month, and so on.