Final answer:
The Julian date refers to the number of days since the start of the Julian Period, not to be confused with the Julian calendar which was later refined into the Gregorian calendar to keep it in line with the solar year.
Step-by-step explanation:
A Julian date is indeed the count of days since January 1, 4712 BC, the starting point of the Julian Period. This system of dating is different from the Julian calendar, which was introduced by Julius Caesar, and later adjusted by Pope Gregory XIII to correct the slippage caused by the fact that the average year in the Julian calendar was longer than the actual solar year. The Julian calendar presumed an average year of 365.25 days, which was an overestimate by approximately 11 minutes of the true solar year. This small error led to a drift over time, culminating in a reform known as the Gregorian calendar, currently in use today, which adjusted leap year rules to ensure the calendar remained closely synchronised with the Earth's revolutions around the Sun.