Humans have been tracking the length of the days, essentially time, since prehistoric times.
The length of a day changes daily. It is like the earth is sitting on a ball. Every day the we move in little increments around the orbit of this ball (our path around the sun)
The reason the days are not equal is that this orbit we travel around the sun is not exactly round in shape. It's squashed. Here on a link to wikipedia it shows our orbit around the sun
and the shape of it.
The shortest coming as I sit here and write, is December 21 2017. Each day will will add one minute of daylight for 17 days. Then it increases daily in tiny increments, by January 7, we will have increased to 2 minutes a day until the longest day, which will be next June 21, 2018.
Since the winter solstice is coming up, I am including a link to winter solstice music.