Convert time units between units, using exact SI factors.
A day holds 86,400 seconds: 24 hours times 60 minutes times 60 seconds. The second is an SI base unit, defined in the BIPM brochure by a caesium atomic transition rather than by the Earth's rotation, so it stays fixed even as the day's length drifts.
Base sixty comes from Babylon, where 60 was convenient because it divides by 2, 3, 4, 5, 6, 10, 12, 15, 20 and 30. The minute and hour are not SI units, but the BIPM brochure accepts them for use with SI precisely because the habit is so entrenched.
A calendar year has 365 days, a leap year 366, and the Gregorian average is 365.2425 days. The astronomical year is 365.2422 days. The year is not an SI unit, so precise work expresses it in seconds per the BIPM definition rather than assuming a round number.
Divide the minutes by 60 and add them to the hours: 7 hours 45 minutes is 7.75 hours. The step matters for pay, because an hourly rate multiplies a decimal, not the string 7:45. Expressed in SI seconds, the same span is 27,900 seconds.