There are two separate reasons for the dates and years being different in different calendars. First, let’s understand the reason why the year is dissimilar. Calendar is an instrument or system that fixes the year’s beginnings as well as its length, and the end. In other words, it is the measurement of time. Hence one has to have some sort of a starting point for doing so. Therefore, some such event is selected which, due to its enormity, can be considered as a starting point of time. Calendars based on religion consider some great event of their particular religion as their point of commencement. The Muslims started counting the years when their great Prophet Mohammed started out his journey (Hijrat). On the other hand, Buddhists counted the year of Gautam Buddha’s death as the point of inception of their calendar. The Jews started their count from the time when God created life on earth. How they knew the first year of life on earth is anybody’s guess. But today, compared to any other calendar the Jews calendar shows the highest number of years (5771) elapsed.
And again, the year can never be divided exactly into months having equal number of days. When one tries to do such a division, there is no number which gives zero remainder. One year is the time taken by the Earth to complete one circle round the Sun and that is equal to 365.25 days. Had the day been over fully, it could have been easily included in the year, but how to fit in 1/4th day? Just to get the calculation right all the calendars have been set differently, due to which there is difference in dates.