Numerous online (or downloadable) Islamic calendar converters can be found on the internet. Most of these converters are based on the tabular Islamic calendar but details of which variant is used is usually lacking. The following list indicates which variant (and epoch) is used in some online (or downloadable) converters.
For some time Microsoft software has included an Islamic calendar converter known as HijriCalendar which sets the date to the Islamic lunar calendar when the language is set to Arabic. In the earliest versions, the algorithm on which it was based was mysteriously referred to as the “Kuwaiti Algorithm” and was described in Microsoft web pages as:
The Hijri calendar is very important to Saudi Arabia and other countries such as Kuwait, and thus this seemingly unsolvable problem must be solved. In an effort to solve this challenging problem, several years ago some of the top developers in Microsoft’s Middle East Products Division (MEPD) did extensive research into it. They had the longest timeline of information on the Hijri calendar as is used in Kuwait, and they took this information and did statistical analysis on it, finally arriving at the most accurate algorithm they could devise.
It can easily be demonstrated that the so-called “Kuwaiti Algorithm” was based on the standard arithmetical scheme (type IIa) which has been used in Islamic astronomical tables since the 8th century CE.
Most online (and downloadable) calendar converters based on the tabular Islamic calendar are able to convert calendar dates before the Gregorian calendar reform in 1582 but often it is not clear whether the thus obtained dates are given in the Julian calendar (as they should be) or in the Gregorian calendar (which is undesirable). This distinction can be important when comparing Islamic calendar dates with contemporary Christian calendar dates as calendar dates before the calendar reform of 1582 are always given in the Julian calendar.
When in doubt, convert the epoch date (1 Muḥarram 1 AH) to the Christian calendar to resolve this issue.