When you click/tap on the words for help something completely different is shown
In romanian the words "ieri", "maine", "alaltăieri", "poimâine" are usually used to indicate the moment of an action. Here the translation for yesterday is "ziua de ieri" because "yesterday" is used as noun.
Could you say ”Ziua de ieri nu este mai!”?
But "ieri nu mai este" was also accepted
By DL I mean