Faith and common belief … it’s the same system that basically works for many of the cultural ideas outside of actual science that we all use. Money, calendars, time, historic cultural units of measurements … all based on belief, habit and repetition.
We don’t know if it is actually 2024 … we just have 8 billion people (maybe 7,999,999,998 if you and I don’t agree) who all believe that it is 2024 that is all.