Length of days is depended on how fast the sun circles above our heads. Time is relative: for the sun to make one circle above our heads, the day is divided into 24 hours: One hour consist of 60 min, which holds 60 seconds: How long a second is, is based on how long it takes for the sun get back to the same place it is in the sky, one day later.

As the sun moves from the northern hemisphere to the southern hemisphere, the distance traveled for the sun in 24 hours increases, while the speed that the sun is circling above our heads also increases. Lots of variables, meaning counting microseconds (0.000001 seconds) is not going to be that accurate throughout the year.

Jesus also prophesised that our days was going to be shortened.

Revelation 8

12 And the fourth Angel sounded, and the thirde part of the Sunne was smitten, & the third part of the Moone, and the third part of the starres, so as the third part of them was darkened: and the day shone not for a third part of it, and the night likewise.

After the sounding of the fourth trumpet, our days has been 1/3 times shorter. Probably happened about 4000 years after creation. Not only did they use to live longer than us, their days was also longer.