The common modern calendar has a long and highly debated past, and this is how it came to be.
In the past, there were several calendars, such as the Hebrew calendar, Egyptian, Mayan, Chinese and even the Roman calendar, each claiming to be more accurate than the last. This actually continues today, so why are we using the one we use?
A calendar is used to help keep track of time and does not necessarily mean it has to be done in a particular way. Some calendars are based on cycles caused by the moon, some by other planets such as Venus, and others on the rotation of the earth around the sun.
The calendar we use today is based on an updated version of the Roman calendar called the Gregorian Calendar, and is now recognised as the international standard representation of time and date tracking together with the international dateline. The Gregorian Calendar is listed by a full orbit of the Earth around the sun, which is divided up into four seasons and then twelve months, which consist of both odd and even days of 24 hours each. As accurate as you might think this calendar is, it is actually more complicated. The total days in a year do not divide up perfectly into 24-hour increments, therefore, every four years, a day is added into the year to make up for this lost time. That's why we have leap years.
Take a look at the video below by YouTube channel, jeremiahjw, on a short history lesson of the modern calendar.