This is the 968th most frequent Hindi word.
aaj
"Today"
'आज' is used to denote 'today' in this sentence, indicating the current day.
आज मौसम बहुत अच्छा है।
Today the weather is very nice.
Here, 'आज' specifies the day on which the action (not going to school) occurred, clearly referring to 'today'.
मैं आज स्कूल नहीं गया।
I did not go to school today.
In this context, 'आज' is used metaphorically to refer to the present time or era.