Reference: Fataawa Noor ‘alad-Darb – Question No.6096, Page 515, Volume 11
Question: If someone swore an oath to do something today, and then [ended up] doing it after the day had passed, is he required to make an expiation?
Response: If someone swore an oath to do something, and stipulated this act be done at a specific time, and the time passed before he got around to doing the act, then [in this case] he is required to make an expiation. For example, if he said ‘By Allaah! I will pray two rak’ahs [of voluntary prayer] today’, and the day passed [without him having done so], then he is required to make an expiation. If, however, he said ‘By Allaah! I will pray two rak’ahs [of voluntary prayer]’ – but he didn’t stipulate any time [when he was going to do so], then he has a choice of doing so now – which is better and safer [so he doesn’t forget], and if he chooses to delay it [for another time] then there is no harm [in doing so].