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.
Some early 20th century and most 19th and earlier centuries coins have the full AH date from the start of a new sultanate/ruler only but many of these coins also feature an additional regnal number (year of reign) elsewhere on the coin e.g. AH + regnal number = AH date of issue.
Bottom = 1223 (year of accession) Top = 29 (regnal year).
Year of issue: 1223 + 29 = 1252 AH = 1836 AD
More modern coins can feature the date of issue in AH and/or AD format, using Arabic and/or Western numbers.