This is the 349th most frequent Estonian word.
aastast
"Since" or "from the year."
In this case, 'aastast' is used in the expression to indicate transitions from one year to the next.
Ta õppis aastast aastasse.
He studied from year to year.
In this sentence, 'aastast' indicates the starting point in time from which the action has been continuing, commonly used when referencing a year.
Ta on siin töötanud aastast 2015.
He has been working here since 2015.
Here, 'aastast' signifies from which year the validity of the law has been in effect.