

Is YESTERDAY a past time?
If yes, I don't know how to define past.
When we were a child and could not analyze anything, we were taught that YESTERDAY is a definite past time adverb (so we use Simple Past). But now we may ask, how to define it as past?
Say, I agree 25April2003 is a past, because it is definitely past and we don't have another 25April2003. It is gone and never comes back anew. However, we will forever have a new YESTERDAY!! If we regard 25April2003 as YESTERDAY, on the next day, we will call 26April2003 as YESTERDAY. On another day, we have another day of YESTERDAY: 27April2003. Even today, we still have YESTERDAY. I guess that tomorrow we may still have another new YESTERDAY. Therefore, YESTERDAY is never gone, and never ended, and thus never past.
If so, how comes we say YESTERDAY is a past time? Ultimately, what is the standard to define a time as past?
Your opinion is welcome.
Xui
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I will remind myself not to deliberately watch messages from some academic figures.