The Hijri calendar as a pure lunar calendar, the time of day change and also the date not midnight as applicable for the Gregorian calendar, the time of day and date for the Islamic calendar through the opinions of the scholars mentioned at maghrib. But this opinion is not specific to the limitation of the day as part of a unit of calendar number called "day" where 1 (one) day is equal to 24 hours. The basis of the Hadith is obtained from the order that if the hilal is visible then that night has entered the month of Ramadan to carry out fasting. The fast itself is carried out tomorrow, every day during the month of Ramadan, from dawn until the next maghrib.
For the duration of one day from Maghrib to Maghrib in the context of worship even though there is a change in time and a long difference from Maghrib to Maghrib every day as a result of changes in the declination angle of the earth and sun every day is not a problem, but calendar calculation is impossible to refer to if time maghrib is different for each coordinate of a place and ranges from a month from maghrib to maghrib somewhere can be reduced or overdue from a month 29 days, or a month 30 days.
A representative calendar must calculate the daily duration, monthly duration and annual duration of the calendar using the standard unit and the standard unit for one day is 24 hours
The transition time at maghrib is associated with the difference in maghrib time in each place because of the declination angle of the earth and the sun which changes at any time, so there must be provisions and arrangements for the determination of the transition of the daily calendar of the Hijri calendar. It is not possible for a calendar to switch time at Maghrib (sunset) even though every place will be different in maghrib time. At longitudes that are high latitude at maghrib, it will get late night than usual as an example for the city of Moscow - Russia longitude 37 37 '4 "BB and latitude 55 45' 21" LU then in June 2019, the maghrib occurs at 21: 11. Thus for the calculation of the Hijri calendar, the determination of the transition hour can be selected at 18:00 (the daily hour of the Gregorian calendar) is to show 00:00 hours of the daily Hijrah calendar. Actually there is no problem if you choose the transition of daily hours such as the Gregorian calendar transition hour at midnight at 24:00 as long as the religious paradigm of the duration from Maghrib to Maghrib is distinguished by the duration of one calendar day.
Al Quran letter 17 verse 12 says that to count the number of years and calendar reckoning is with two signs day and night on earth. Signs day and night on this earth are not measured alternately from maghrib to maghrib every earth coordinate but measured by the maghrib boundary between day and night with the same length of time which is 12 hours each. This is the basis for the calculation of the year and calendar reckoning, which is the same day and night, which can be set from 00.00 midnight or 18:00.