"It's just a convention" This is correct. In fact, in one sense our clocks could be argued to be minutes out, on average: originally time was measured by the s

"It's just a convention" This is correct. In fact, in one sense our clocks could be argued to be minutes out, on average: originally time was measured by the sun. But by the sun, the earth doesn't rotate in exactly the same time each day during the year. So a clock assuming each day is the same length (like they all do) accumulates an error with respect to sun time of more than 16 minutes,and then loses it again, in a cycle. But we just collectively decided that it was simpler not to bother with that. We changed the convention so that time is marked with respect to an average day length