The Turkish Historical Society Date Conversion Guide is a software developed and made available by the Turkish Historical Society to perform date conversions between Gregorian, Hijri, and Rumi calendars. It is accessible online. The program converts historical data entered into other calendar systems based on mathematical and astronomical calculations.
TTK Date Conversion Guide (TTK)
Principle of Use
The program performs date conversion in three levels:
1. Year-level conversion: When only the year is entered, its approximate equivalent in other calendars is calculated.
2. Month and year-level conversion: When month and year information is entered, the equivalent of the relevant month in other calendars is calculated.
3. Day, month, and year-level conversion: When a full date is entered, the equivalents of the entered date in other calendars are calculated with a high level of accuracy.
Conversion results are usually presented in a table with a deviation margin of ±1 day.
Features and Limitations Related to the Hijri Calendar
Due to the structural characteristics of the Hijri calendar, month beginnings are based on the "ru'yet-i hilal" (sighting of the crescent moon) criterion. This reveals the impact of meteorological conditions, in addition to the moon's position in the sky, on the calendar. The Date Conversion Guide, however, operates on astronomical principles, i.e., based on the periodic movements of the moon and Earth. For this reason, especially for historical Hijri dates, the Gregorian equivalent may show a deviation of ±2 days.
The program determines the beginning of months based on the mathematically calculated lunar cycle, rather than the possibility of the crescent moon being seen with the naked eye. Therefore, in situations where historical events need to be determined with day-level precision (e.g., the destruction of Takiyüddin Observatory on Thursday, 4 Dhu al-Hijjah 987), the researcher should combine the approximate Gregorian equivalent with the day of the week the event occurred to arrive at the exact date.
Rumi Calendar Conversion
There is no such uncertainty in date conversions related to the Rumi calendar. Since the Rumi calendar progresses with a fixed difference from the Gregorian calendar, conversion results can be obtained linearly and reliably.
Calculation of Religious Days and Ru'yet-i Hilal
The "ru'yet-i hilal" criterion, which is essential for determining the beginnings of Hijri months, is crucial for identifying religious days, and today this process is carried out by the Presidency of Religious Affairs in Türkiye. The Presidency announces religious days based on the visibility of the moon, using data from Kandilli Observatory. In contrast, the TTK Date Conversion Guide is a tool prepared not for calculating future religious days, but for converting past dates.
Date Conversion Table (TTK)
Scientific Reliability and Purpose of Use
The date conversion program developed by the Turkish Historical Society has been tested on millions of examples to support historians' scientific work processes and has reached a high level of reliability. However, using the program for purposes other than its intended use, such as religious calendar calculations or daily use, may lead to misleading results. Therefore, the user must consider the historical context and the characteristics of the calendar.
