The day begins when the first rays of the sun becomes visible in the eastern horizon. Traditional Arabs measured the times of the day from this moment. It is still followed in the African countries, which were once under Arab rule. In Makkah the day starts at Sahar. Sahar is at GMT 00.00 or 3 MST. It is 0.00 hour at Makkah. The days are identified by their names when they are within a week. They are only seven and are repeated every week and in longer periods of months they are identified by numbers which we call “date”.
Just as a day in a week cannot have two names, a day in a month cannot have more than one number. These numbers are repeated month after month. It therefore follows in the same manner as a day is known all over the world by its unique name the day should also be identified by a unique number, in orders that everyday is recognized all over the world by the same name or number (date). There is just one problem here - how do you reckon the date or the time period to be called a Day by a name or a date. Navigators of the bygone days had the same problem. Arab navigators had two constraints before them. The first was that they had always to turn their faces towards Kaaba for their prayers. Sailors traveling towards east with faces towards west will continue in the same direction until they meet sailors traveling towards west with their Qibla in the east.
At the point where they meet, the two sets of sailors would be facing in opposing direction – that is there will be two Qiblas for the people at the same place. This is not proper. One place can have only one Qibla. The only way to rectify this situation was for everyone to choose one direction or Qibla and that means it is necessary to identify a turn around point. People living in places on the west of this turn around point will face eastwards and those on the eastern side will face westwards in order to keep Kaaba as the Qibla for both. The confusion is for those on the point or where this point falls in a populated village or town. The people there would be tempted to look towards both directions. Islam does not permit two Qiblas for the same town.
The second constraint is that it is also an Islamic rule that there cannot be two Jumu’as in the same week or same place. Thus when the countries west of Samoa Island observes Jumu’a prayers, Samoa and countries east of it will be offering Zuhar prayers of Thursday. Because the Islamic system of daily prayers begins with the decline of the sun from mid-heaven, the date change happening at this time on the date line is perfectly in line with its requirements.
A north south imaginary line was therefore fixed in such a manner that it passed through uninhabited place covered with water or sea between the islands of Fiji and Samoa in the eastern hemisphere. Though slightly zigzag it met both the constraints. The European travelers later adopted this line as The International date line. This line closely coincides with the Greenwich (0 Meridian) line on the opposite side. The International Date Line thus serves as a permanent turnaround point for the Qibla and the Jumu’a.
At a place where the Hilal is visible its appearance is just after the sunset. The moon would set about 12 minutes later than the sun at that place. At other places it may occur at any time of the day or night. The month begins immediately after the emerging of the moon from the Sun. But a day can start only with sunrise. So it follows that the first day of the lunar month will be the day following the day on which the Hilal occurs.
At Greenwich the corresponding time is 0:00 hours i.e. midnight. When we reckon universal time a day starting at 0:00 hours though the day change occurs when the sun passes on International Date Line at mid-day, it is accounted from the point of time it reaches 0:00 hours at each geographical point.