Julian Date Converter
Pre-Julian research model
Roman Republican Date Converter
Convert Gregorian dates to a schematic Roman Republican calendar with a Mercedonius intercalary month model, or convert model dates back to Gregorian.
Live output
Result
Choose a direction, enter a Gregorian or Roman schematic date, and the model result appears here.
Guide
Roman Republican Date Converter for a schematic pre-Julian model
This Roman Republican Date Converter changes Gregorian dates into a schematic pre-Julian Roman Republican calendar model and converts model dates back into Gregorian dates. The model includes traditional Roman month names and a Mercedonius intercalary month marker.
The historical Roman Republican calendar did not follow a simple mechanically predictable intercalation in every year. This page therefore labels its calculation as schematic and does not claim to reconstruct every political or documentary intercalation.
What this converter handles
Traditional Roman month names with a visible intercalary model
The converter uses Julian Day Number arithmetic as the internal bridge. Roman Republican years are counted from the configured epoch, regular month lengths follow the page model, and even model years include Mercedonius as month 13.
Reverse conversion validates the intercalary suffix. A Mercedonius date must use month 13 and the -I suffix, and it is accepted only in years that are intercalary in this schematic model.
Gregorian to Roman Republican model
Enter a Gregorian date and return the Roman model year, month name, day, year type, and reference JDN.
Roman model to Gregorian
Enter YYYY-MM-DD for regular months or YYYY-13-DD-I for Mercedonius in an intercalary model year.
Pre-Julian caveat
The page separates repeatable model conversion from historical reconstruction of irregular Roman intercalation.
How to use it
Use -I only for Mercedonius input
Choose Gregorian to Roman when your source date is Gregorian. Choose Roman to Gregorian when your source is already written in this page's schematic Roman Republican numeric format.
Regular input uses YYYY-MM-DD. Mercedonius input uses month 13 plus the -I suffix. The converter rejects month 13 without -I and rejects Mercedonius in non-intercalary model years.
Convert from Gregorian
Enter a date such as 2026-06-30 to see the schematic Roman month, day, and year type.
Convert from Roman Republican input
Enter values such as 2052-01-17 for regular months. Use -I only when entering month 13 Mercedonius.
Keep historical claims narrow
Use the result as this page's model output, not as a final authority for ancient Roman documentary chronology.
Reference examples
Reference conversions from the local implementation
These examples are produced by the same local model functions used by the form. They show the regular month syntax and the reference JDN shared by both directions.
Gregorian to Roman Republican
The Gregorian date 2026-06-30 converts to Roman Republican model date 2052-01-17, shown as Martius 17, Year 2052, with reference JDN 2461222.
Roman Republican to Gregorian
The Roman Republican model date 2052-01-17 converts back to Gregorian 2026-06-30 in this schematic pre-Julian model.
FAQ
Roman Republican conversion questions
Roman Republican calendar conversion is difficult because historical intercalation could be political and irregular. These answers define the limited arithmetic model used here.
Does this reconstruct actual Roman intercalations year by year?
No. It uses a schematic model. Real Republican intercalation history can require documentary and chronological evidence outside this converter.
What does -I mean?
-I marks Mercedonius, the model intercalary month. It is valid only with month 13 in model intercalary years.
Is this the Julian calendar?
No. The Julian calendar is a later reformed calendar with different leap-year rules. Use the Julian Calendar Converter for Julian calendar dates.
Reference
Calendar implementation reference
The implementation follows the absolute-day method used in calendrical calculation references while keeping the model caveat visible for a historically irregular calendar.