O People, the deniers of Allah's power indulge in tampering with the calendar in order to make permissible that which Allah forbade, and to forbid that which Allah has made permissible. The time has now returned to the position as it was when Allah created the heavens and the earth with a year of twelve months. Four of them are sacred: Dhul Qi'dah, Dhul Hijjah, and Muharram; and Rajab, which lies between Jumada and Sha'aban.
Among the challenges given to us by Allah is the implementation of the Islamic timekeeping system. Allah's Messenger Muhammad, may peace and blessings be with him, announced during his farewell message that the practice of adding or subtracting time (intercalation) from the calendar is unlawful, and proceeded to establish the Islamic calendar as the only official calendar.
The timing of this event is significant. It ended the practice of adjusting the days between the two dominant calendars of this period -- the Meccan lunar-solar calendar and the solar calendar -- in the year when the Meccan lunar-solar calendar had just been adjusted according to usual intercalations. It was after this adjustment had been made Allah revealed that months had returned to their original position.
This means that today when we refer to Ramadan or any other month on the Islamic calendar, the month is the exact same month that occurs at the right time and in the right sequence as when Allah created the heavens and the earth. Given the billions of years between the creation of the earth and the creation of humanity, as well as all the unknown events where time was adjusted according to different people's understanding, only Allah has the knowledge to make this statement with absolute truth.
Prior to this revelation, the Prophet, may peace and blessings be with him, used the solar year by means of actual harvests for collecting agricultural taxes. For other purposes he applied a purely lunar year. Yet, when he relayed the message during the pilgrimage in 10 A.H. and from that time forward, he used exclusively the Islamic calendar.
Following the death of the Prophet, may peace and blessings be with him, the Islamic calendar remained the only official calendar used by Abu Bakr, 'Umar, 'Uthman, and 'Ali. Not only was it adopted by them, but also it was improved by their use. Adoption of Muharram as the first month is attributed to 'Uthman. The adoption of the Hijrah as the starting point, is variously attributed to 'Umar, 'Uthman, and 'Ali.