30 january is the number of days in a julian calendar year.
how many days in a julian calendar year?
30 january
although greek astronomers had known, at least since hipparchus, a century before the julian reform, that the tropical year was slightly shorter than 365.25 days, the calendar did not compensate for this difference. as a result, the calendar year gains about three days every four centuries compared to observed equinox times and the seasons. this discrepancy was largely corrected by the gregorian reform of 1582. the gregorian calendar has the same months and month lengths as the julian calendar, but, in the gregorian calendar, year numbers evenly divisible by 100 are not leap years, except that those evenly divisible by 400 remain leap years. (even then, the gregorian calendar diverges from astronomical observations by one day in 3,030 years).
julian calendar
the determination of leap years in the proleptic julian calendar (in either numbering) is distinct from the question of which years were historically considered leap years during the roman era, due to the leap year error: between 45 bc and ad 8, the leap day was somewhat unsystematic. thus there is no simple way to find an equivalent in the proleptic julian calendar of a date quoted using either the roman pre-julian calendar or the julian calendar before ad 8. the year 46 bc itself is a special case, because of the historical introduction of the julian calendar in that year, it was allotted 445 days. before then, the roman republican calendar used a system of intercalary months rather than leap days.
proleptic julian calendar