It was the Roman calendar.
They used the Egyptian Calender.
History does not have a written record of who first used "before Christ". You can figure it could have been a monk or Catholic priest because they started up the calendar with AD and BC designations.
Assuming that you want a copy of the calendar used in the Western world, you can type 'petpetual calendar' in to a search engine. It will display calendars back to about the 1500's, if not futher back.
soapstone and concrete
There was a change from the Julian to the Gregorian calendar in September 1752. Newton was born on 25 December in the Julian Calendar that he would have used. This is equivalent to the 4 January in our calendar.
Easter is set according to phases of the moon and not by date, this is why it varies. ANSWER Historians are not as certain about the birth of Christ in relation to the Hebrew calendar. Easter is more certain and then can be transfered to the standard calendar used by the modern Western world. Christmas, in addtion to the birth of Christ, is a huge boon for retailers. For convenience reasons, it is a set and well established date. In reality, Christ's birth would vary like Easter because his birth was marked on the Hebrew calendar. The calendars do not transfer exactly. Easter is a good example.
February 11 was his birthday on the old style calendar that was used when was he born. Supposedly he continued to celebrate his birthday on Feb 11 even after the calendar was changed.
There was an old Germanic calendar, but it was different to the Roman calendar, which is now used. The Germanic calendar no longer is used.
Nanakshahi Calendar.
Google Calendar is a calendar that can be used and viewed online.
AD is not used in the Jewish calendar. It is only used on the Gregorian (Christian) Calendar.
BC is before the Christian era; it is used in dates before the supposed year Christ was born.