Well, Roman Imperialism. The sheer reach of the Roman Empire is the reason why it became a de facto standard, difficult to displace. The month names are still Roman, and the days of the week are pagan gods.
https://en.m.wikipedia.org/wiki/Global_Positioning_System#Ti...
That the years are structured around the death of Christ... Eh? Hard to say. I believe that was retro-fitted, and probably was more centered around Augustus's rise to power and Rome moving from a republic to an empire. Or maybe not, I haven't read the relevant wikipedia page for that.
EDIT: I meant to say that the Julian calendar did not always use BC/AC but adopted it some time prior to the invention of the Gregorian calendar.
The motivation was to stop the drift of the whole Christian festival calendar towards later and later days in the year. We have some confusion about Easter in contemporary accounts of the reform, but the date of Easter when calculated as the first Sunday after the first full moon after the spring equinox is independant of any calendar system. This would have implied that all other festivals, based on the Julian Calendar, would have caught up and then overtaken Easter (and caught up again, etc.).
> and to ensure that people around the world were celebrating it at the same time.
This was definitely not the intention. All people in the Protestant/Catholic/Orthodox "realm" were already using uniformely the Julian Calendar. Differences of the Easter date were due to different algorithms for its calculation. In practice, on the contrary, it led to more differences, because the reform was not accepted for a long time in many parts of the Christian world, even in Catholic ones.
> While you are definitely right that the Gregorian calendar stems from Roman culture, one cannot deny that its revisions were religiously motivated and funded.
Well, the flaws of the Julian System had a consequence for the Christian festival calendar. But to correct this flaw was also a very reasonable thing to attempt. We are in in age were astronomy (and astrology) become a very popular science.
> Further, that its history and global adoption is intertwined with the spread of Christianity around the world.
This is not true. For example in what become later the British empire the Julian Calendar was in use until 1752. So for many areas the adoption come long after Christianity took roots. And then it was implemented in areas that never become widely Christian, such as India.
> With its global adoption cultures all around the world have lost their connection to their own calendars and practices.
Most did not loose connection. It become rather dual use.
> As for the BC/AC part I believe it was always part of the Gregorian calendar? The Julian calendar also used BC/AC. :)
> EDIT: I meant to say that the Julian calendar did not always use BC/AC but adopted it some time prior to the invention of the Gregorian calendar.
"BC/AC" is wrong, it is: "BC/AD" ("Before Christ" and "Anno Domine" = "In the year of the Lord").
The Gregorian Calendar strictly speaking applies only to dates after AD 1583. If you extent it backwards (avoiding the date gap in 1583), it is called the "proleptic Gregorian Calendar". There exist different version of it. For example ISO 8601 uses a proleptic Gregorian Calender with a year zero.
Besides first appearances in the late 15th century, Julian Calendar BC dates started to be widely used only much later than the Gregorian reforms. When writing about ancient history, authors used AUC dates ("Ab urbe condita" = "From the founding of the City [Rome]" = 753 BC) or dated from the (suppost) beginning of the world, thus avoiding negative numbers.
But much before that, Jewish-Christians were not exactly friendly to Romans/"pagans", and vice versa. To put it lightly. Clash of civilizations: completely different values.