The main purpose of a calendar is to calculate and keep track of time. Yet, Muslims use the Hijri Calendar, Christians use the Gregorian Calendar, and Jews use their own calendar. Which one of them is right? It turns out that the correct calendar is God’s calendar (10:5, 55:5); it’s written in the sky, and according to this correct calendar we use the sun to calculate the years, and we use the moon to calculate the months (10:5).
So, the Christians have changed the months incorrectly, and the Muslims have changed the years incorrectly, because they both wanted the months and the years to synchronize with each other. But they don’t have to synchronize. Do we also change the weeks so that they synchronize with the months? Do we make the weeks longer, or the months shorter? No.
A week is 7 days, a month is 29 or 30 days, and a year is 365 days. This means that one year has 12 months. So, the Christians have got it right when it comes to the years, the Muslims have got it right when it comes to the months, and the Jews have got it right when it comes to the weeks – their Sabbath is the seventh day (4:154).