Is the criterion in his (peace and blessings be upon him) saying, “I was supported by fear for a distance of one month’s journey,” the distance or the time?

Question : The Prophet’s (peace and blessings be upon him) saying that the journey is that of a pedestrian for one month: means of transportation differ between the past and the present. Is the criterion the distance or the time?

Answer : It appears that the criterion is the distance, because he said “a distance of one month’s journey,” so it is understood according to what was customary in his (peace and blessings be upon him) time.