Yes, the Islamic day starts after Magrib.

The night starts at maghreb 24/3 and ended at fajr 25/3

Firstly: There is no report in the Prophet’s Sunnah to suggest that there are specific surahs or verses or adhkaar which, if the living recite them over the dead, they will have any effect such as making the deceased steadfast when he is questioned in al-barzakh, or that they will protect the deceased from the punishment of the grave. That which benefits the deceased most at that time, of actions on the part of the living, is their offering supplication (du‘aa’) for him and praying for forgiveness for him. There are a number of answers on our website which highlight the weakness of all the reports that have been narrated about the virtue of reciting Surah Yaa-Seen in particular. Some people believe that this surah has a particular effect in making things easier for the deceased in his grave, and they recite it over the grave after the burial, even though there is no proven report to support that in the Prophet’s Sunnah. Please see the following answers: 6460 , 82800 , 75894 and 72201 Secondly: According to Arab custom and Islamic teaching, the night belongs to the day that comes after it, not the day that comes before it. There is a great deal of evidence to support that, but the clearest evidence is what the people say on occasions of worship, such as the following: -1- Sighting the new moon of Ramadan, which indicates that the blessed month has begun. It may be noted that this night is regarded as part of Ramadan and the people pray Taraweeh on that night. -2- Sighting the new moon of Eid al-Fitr, which indicates that Ramadan has ended and the month of Shawwaal has begun. On that night the Muslims stop praying Taraweeh. When an Islamic text mentions the night of Friday, or the night of Eid, and so on, what is meant is the night according to the Islamic custom mentioned above. The same applies with regard to Laylat al-Qadr. There is a saheeh hadeeth which clearly indicates that the morning belongs to the night that comes before it. This is the hadeeth of Abu Sa‘eed al-Khudri (may Allah be pleased with him) according to which the Messenger of Allah (blessings and peace of Allah be upon him) used to observe i‘tikaaf during the middle ten days of Ramadan. He observed i‘tikaaf one year then, on the night of the twenty-first – which was the night on the morning of which he would come out of his i‘tikaaf – he said: ‘Whoever was observing i‘tikaaf with me, let him observe i‘tikaaf for the last ten days, for I was shown this night (Laylat al-Qadr), then I was caused to forget it. I saw myself prostrating in water and mud on the following morning (lit. on its morning). So seek it in the last ten days, and seek it on every odd-numbered night.” It rained that night, and the mosque was covered with palm branches and the mosque leaked, and my own eyes saw the Messenger of Allah (blessings and peace of Allah be upon him) with marks of water and mud on his forehead on the morning of the twenty-first. Narrated by al-Bukhaari (2027) and Muslim (1167) Think about the words “the night on the morning of which he would come out” and “on the morning of the twenty first”– this indicates that the morning belongs to the night that has just passed. Imam al-Qurtubi (may Allah have mercy on him) said, in his commentary on the verse in which Allah, may He be exalted, says (interpretation of the meaning): “And We appointed for Moosa (Moses) thirty nights and added (to the period) ten (more), and he completed the term, appointed by his Lord, of forty nights” [al-A ‘raaf 7:142]: The verse also indicates that the date starts with the night, not the day, because Allah, may He be exalted, says “thirty nights”. That is because the night is the beginning of the month. This is also how the Sahaabah (may Allah be pleased with them) spoke of days. It was narrated from them that they used to say: We fasted five with the Messenger of Allah. But the non-Arabs did it differently and based their counting on days, because their calendar was solar. End quote from al-Jaami‘ li Ahkaam al-Qur’an (7/267) Thirdly: Thus it is clear that if a person dies at 1 a.m. on Saturday, then according to Islamic custom he died on “the night of Saturday” and not on “Friday night.” So he is not included in the marfoo‘ hadeeth of ‘Abdullah ibn ‘Amr ibn ‘Aas, “There is no Muslim who dies on the day of Friday or the night of Friday but Allah will protect him from the trial of the grave.” Narrated by at-Tirmidhi (1074); classed as hasan by al-Albaani in Saheeh at-Tirmidhi However it should be noted that there is a difference of opinion among the scholars of hadeeth as to whether this hadeeth is saheeh or da‘eef. The majority are of the view that it is da‘eef. For more information on its isnaad, please see the following link: http://www.alukah.net/publications_Competitions/1008/38 And Allah knows best.

In Islam, a new calendar day starts with Magrib.

I am not aware of any country that syncs clocks with the time for maghrib.

Apart from the immense confusion that such a practice would create, it would also require specialized clocks or periodic resetting since the time from one Maghrib to the next Maghrib is not 24 hours, but usually something like 23 hours 58 minutes or 24 hours 2 minutes, it constantly shifts throughout the year, going backwards in winter or forwards in summer.

A 'theological justification' for doing so is not needed as there is no obligation of keeping time in hours nor is there any religious matter in Islam that is dependent on a measure of time in hours.