Calendar Convert
Convert dates between Gregorian, Julian, and other historical calendar systems — essential for working with ancient birth data in astrological software.
This tool matters because many astrological programs handle historical dates differently — some require Julian input, some convert silently, and some get it wrong entirely. Before entering a historical birth date into any software, it's worth confirming which calendar system it expects.
Practical example: Hildegard von Bingen's commonly cited birth date is August 17, 1098 — but that's in Gregorian, a calendar that didn't exist until 1582. The Julian equivalent is August 11, 1098, a 6-day difference that can noticeably affect the chart. Writing to a software developer can tell you which system they use; this tool tells you what to input.
Additional calendar systems across time, space, and cultures are also included.
Enter a date in any calendar above and it is converted into all the others at once. Years use astronomical numbering, where year 0 = 1 BCE, −1 = 2 BCE, and so on. Calendars are shown proleptically — extended backward before their historical introduction — so any date can be compared across systems. A few notes on the wider set: the Islamic date uses the tabular (arithmetic) scheme and may differ by a day from sighting-based civil calendars; the Persian and French Republican dates use their arithmetic algorithms; and the two Old Hindu calendars (Ārya system) use the mean sun and moon fixed to the Kali Yuga epoch, so they approximate — rather than exactly reproduce — the modern dṛk (true-position) pañcāṅga. The Old Hindu Solar months are the sidereal rāśis. All calculations run locally in your browser — no external service.