Coins using the AH (Anno Hegirae) system date from the time in 622 AD when Mohammed "fled" from Mecca, escaping to Medina. The Islamic calendar since then has used the lunar year to calculate dates. The calendar is approximately 11 days short of the Gregorian (AD) system so there is no fixed formula for calculating the Christian year. AH to AD conversion tables are available.
Some early 19th century coins are undated. All other coins from the early 19th century up to 1925 have the full AH date of the year of issue.
From 1925 to 1976 the SH year (based on a Solar Hegirae year). SH year + 621 = AD year.
In 1977 and 1978 the MS date (Monarchial System) was used.
In 1979 the SH year was re-introduced.