在早年,每個月份的天數並不一致,需要觀測得到新月才能確認新一個月份的到來[78]。《古蘭經》有云:「新月是人事和朝覲的時計。」[89]先知穆罕默德亦指齋月的開始和結束應以新月為準,他解釋稱這是最直觀的方法,不需要曆法運算[90]。學者賈薩斯(阿拉伯语:أبو بكر الجصاص)認為先知規定了確認賴買丹月來臨的方法只有一個,那就是肉眼觀測得到新月。如果受到不利的氣候條件影響而未能在舍爾邦月的29日觀測得到新月,那麼舍爾邦月就會延長到30日。[91]同理,賴買丹月的天數可能是29天或30天,無法預先得知其始終[92]。不過,通過天文計算的伊斯蘭曆法把賴買丹月的天數定為30天[93]。
伊斯蘭曆以月相為根基,每個月份新月出現的時間在全球各地都不一樣,各地的穆斯林應用各種方式來確認日曆。隨著現代穆斯林遍及全球各地,原本自成一角的社群走向環球,他們在確立宗教節慶日期時都會用到伊斯蘭曆,但如何觀測新月的到來成為了一個爭議。許多人都認為只能用肉眼來確認,但有另一些人認為最好的方法是天文測算。全球穆斯林有需求根據伊斯蘭教法和現代科學應用一個可靠、劃一的伊斯蘭曆。[128][129][130]埃及學者艾哈邁德·穆罕默德·沙基爾(埃及阿拉伯文:احمد محمد شاكر)在1939年便稱「全球伊斯蘭陰曆月份必須要調整到同一天開始,這是不可動搖的」[131]。
為了在文化上團結穆斯林社群,伊斯蘭合作組織早在1978年便成立委員會研究統一伊斯蘭曆,但進展有限,主因是沙特阿拉伯堅持要基於麥加的肉眼觀測結果[135]。沙特阿拉伯所用的日曆稱為烏姆古拉曆(阿拉伯语:تقويم أم القرى),從1999年開始,該國開始使用預期能見新月的基準來運算日曆,並獲得伊斯蘭合作組織的認受[136][132]。烏姆古拉曆獲得卡塔爾、巴林及一些非穆斯林國家的穆斯林少數族群廣泛使用[137]。
中世紀伊斯蘭世界的百年翻譯運動把希臘、波斯、印度的著作翻譯成阿拉伯文,這些翻譯作品往往都採用伊斯蘭曆,讓史學家得以追溯知識跨文化傳播的歷程[174]。整個伊斯蘭世界都得以在歷史長河當中採用伊斯蘭曆,令伊斯蘭歷史的紀年得以保持一致[175]。基督教史學家希拉波利斯的阿加皮烏斯(阿拉伯语:محبوب بن قسطنطين)編寫了一部涵蓋至10世紀的史著,當中採用了伊斯蘭曆來剖析伊斯蘭歷史[176]。薩非王朝的一些編年史家也使用伊斯蘭曆的紀年來書寫歷史[177]。
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