by [time-phrase] means "as of" [time-phrase], that is, not later than [time phrase].
The phrase is often used with situations where something is expected to happen, or when someone is waiting for something to happen, or waiting for something to arrive, or warning about something that may happen:
The shipment is supposed to arrive by [the end of next week].
The vendor promised the shipment would arrive by [October 15th].
The thaw of the glacier is predicted to occur by .
The payment is due by [October 1st].
The train will arrive by [11AM].
The rat population in the city is expected to have tripled by [autumn 2025].
NOTE: The only numerical calculation involved is chronological. Any other form of calculation (e.g. population, temperature, cost, rate of change, etc) will be expressed by some other phrase(s) in the sentence.
P.S. Here is how it should not be used, taken from the label on a perishable food product that I purchased recently:
That should say "fresh until September 22".