This recipe relies on the fact that date/time values in DAX are stored as decimal numbers where the integer portion (left of the decimal point) represents the number of days while the decimal portion represents fractions of a day. Hence, for example, 1 second would be stored in DAX as 1 / 86,400 = 1.157407407e-5 (0.00001157407407), where 86,400 is the number of seconds in a day.
Hence, to add hours, we simply need to multiply the number of hours to add by 1/24 = 0.041666667. To add minutes, we simply multiply the number of minutes to add by 1/24/60 = 0.00069444444. To add seconds, we simply multiply the number of seconds to add by 1/24/60/60 = 0.00001157407407.