I tried to comfort Jan by saying:
"I will let you know once she receives the card. Don't worry! The postmen may have been very busy."
I understand that "I will be very busy tomorrow" is alright,
so is "The postmen will be very busy tomorrow".
Now given the following two sentences (1) and (2):
(1): The postmen may be very busy
(2): The postmen are very busy
I am not sure if it is grammatically incorrect by just saying "The postmen may be very busy" or "The postmen are very busy" without a single-word time adverb.