Abu Qatadah narrated that the Prophet, upon whom be peace, Said: «Fasting the day of ‘Arafat expiates the sins of two years: a past one and a coming one. And, fasting the day of ‘Aashura’ (the tenth of the lunar month of Muharram) expiates the sins of the past year».
(Reported by Muslim 1162)
Then means that fasting 12 hours = absolving 24 months.
As the day has 12 hours, then fasting one hour a day absolves two month. Consequently, each 60 minutes = 60 days. Thus, fasting a single minute absolves a whole day.
Fasting a minute on Arafah Day absolves the sins of a whole day spent in doing sins without wasting a minute in other than sins. This means that 86400 (which are the number of minutes a day) are absolved by fasting just one minute on this auspicious day.