Origin of calendar dating
)—when you've become accustomed to rising after the “crack of noon” and doing most of your work when the Sun is down, you appreciate recording your results in a calendar where the date doesn't change in the middle of your workday.
While one can't properly speak of “Gregorian dates” prior to the adoption of the calendar in 1582, the calendar can be extrapolated to prior dates.
As in the Julian calendar, days are considered to begin at midnight.
The average length of a year in the Gregorian calendar is 365.2425 days compared to the actual solar tropical year (time from equinox to equinox) of 365.24219878 days, so the calendar accumulates one day of error with respect to the solar year about every 3300 years.
The calendar thus accumulates one day of error with respect to the solar year every 128 years.
Being a purely solar calendar, no attempt is made to synchronise the start of months to the phases of the Moon.