Adam Clarke on Revelation 9:15

For an hour, and a day, and a month, and a year - We have in this place a year resolved into its component parts. Twenty-four hours constitute a day, seven days make a week, four weeks make a month, and twelve months make a year. Probably no more is meant than that these four angels were at all times prepared and permitted to inflict evil on the people against whom they had received their commission. There are some who understand these divisions of time as prophetical periods, and to these I must refer, not professing to discuss such uncertainties.