In Islamic terms, Dayn (debt) means for one to owe money or a commodity to another, which can be caused by borrowing or other things that one chooses to do such as give a debt as his wife's dowry or give a debt as rent instead of money. Sometimes one becomes in debt because of things that have nothing to do with his choice, such as being responsible for his wife's sustenance (once one gets married, he is responsible for his wife's sustenance, whether he likes it or not) or cases of surety. In cases like these, paying the debt off is obligatory, therefore when the fixed repayment time arrives, it is wajib to pay off the debt.[1]

Concerning your question about paying off debts in the evening or after dark, these are the answers received by the offices of some Grand Ayatullahs:

The office of the Grand Ayatullah Khamenei:

There is no problem in doing so.

The office of the Grand Ayatullah Sistani:

There is absolutely nothing wrong with doing so and what you have been told isn’t true.

The office of the Grand Ayatullah Makarem Shirazi:

There is no problem in doing so.