One can be curious to observe that the Islamic calendar year is about 11 days shorter than the Gregorian calendar year, so there will come a point when these calendars will intersect each other.
So, Islamic and Gregorian are not going in parallel as they will cross each other eventually.
I have decided to shed some light on this and the following is the maths:
The beginning of the Islamic year moves through all seasons in about 34 years’ time, and the difference between the Gregorian year number and the Islamic year number decreases by one every ~34 years.
The Islamic and the Gregorian calendar will INTERSECT around the 5th month of the year 20,874 of the Gregorian calendar.
i.e., the 5th month of the year 20,874 in the Gregorian calendar will start on the same day as the 5th month of the year 20,874 in the Islamic calendar.
We are very unsure if there will be human-being existence to see this intersection and passing-by of the Islamic calendar.