The following are two date calculators. One for the comparison of the difference between two dates, the other for the purpose of add or subtract days, weeks, months, and years from a date.
Number of years, months, weeks, and days between two dates
Add to or subtract from a date
History of Date Calendar in Western World
In the Western world, the Roman or "pre-Julian" date calendar was created at the earliest times of Roman civilization was based on the orbit of the moon. The date calendar is believed to have originally consisted of some months that were 29 days long and other months that had 30 days.
The date calendar started the year in March and consisted of 10 months, with 6 months of 30 days and 4 months of 31 days. The winter season was not assigned to any month, so the calendar year only lasted 304 days with 61 days unaccounted for in the winter.
The 304-day Roman date calendar didn't work for long because it had no clear relationship to the seasons. The Roman King Numa Pompilius reformed the date calendar around 700 BCE by adding the months of January (Ianuarius) and February (Februarius) to the original 10 months, which increased the year's length to 354 or 355 days.
The addition of January and February meant that some of the months' names no longer agreed with their position in the calendar (September - December). The month Quintilis was renamed July in honor of Julius Caesar in 44 BCE and Sextilis was renamed August in honor of the Emperor Augustus in 8 BCE.
The Roman calendar was still flawed after adding January and February, as well as the days and months needed to keep the calendar in line with the seasons. Many attempts were made to align the calendar with the seasons but all failed. An extra month was added to the calendar in some years to make up for the lack of days in a year.
When Julius Caesar became pontifex maximus (chief prelate of the Roman religion) in 46 BCE, he reformed the calendar by getting rid of the extra month. The Julian calendar was created, then completed during his successor Augustus' reign. It had 12 months. Some days were added to certain months, and the total number of days in the year went to 365 from 354. Every fourth year was made a leap year, with an extra day in one month.
But there was still a discrepancy between the true year and the Julian calendar year. In the year 1582, it was observed that there was a 10 day differential. Pope Gregory XIII dropped 10 days from the year 1582 so that October 4, 1582 was followed by October 15, 1582 (the Pope was following the instructions of a great Italian savant named Luigi Lilio). In addition, a modification was made that century years that were not divisible by 400 would not be considered as leap years. For example, 2000 would be a leap year while 2100 would not. This made the year sufficiently close to the actual year and this calendar is called the Gregorian calendar.
However, there is no longer an accurate correspondence of the calendar with the phases of the moon.
Although it is far more accurate than the Julian calendar, the Gregorian calendar is off by about 1 day every 3236 years.
In English-speaking countries in the West now, we all learn the basic folk rhyme to remember dates. The date calculators above were based on the calendar defined below.
"Thirty days hath September,"
April, June, and November.
All the rest have thirty-one,
Excepting February which has twenty-eight,
And in leap-year, twenty-nine."