You may want to provide reference as to where you got this.
Because:
It starts when a person has prayed ‘Isha’, even if it is joined to Maghrib at the time of Maghrib, and lasts until dawn begins, because the Prophet (peace and blessings of Allaah be upon him) said: “Allaah has prescribed for you a prayer (by which He may increase your reward), which is Witr; Allaah has enjoined it for you during the time between ‘Isha’ prayer until dawn begins.” Narrated by al-Tirmidhi, 425; classed as saheeh by al-Albaani in Saheeh al-Tirmidhi.
Once dawn begins, the time for Witr is over, based on the words of the Prophet (peace and blessings of Allaah be upon him): “Pray Witr before morning comes.” Narrated by Muslim, 754.
Muslim also narrated from Ibn ‘Umar (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “Hasten to pray witr before dawn.”
And Muslim narrated (752) that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Witr is one rak’ah at the end of the night.”
Al-Tirmidhi narrated (469) from Ibn ‘Umar (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “When dawn comes, all the night prayers and Witr are over, so pray Witr before dawn comes.” Classed as saheeh by al-Albaani in * Saheeh al-Tirmidhi.*
Al-Bukhaari (472) and Muslim (749) narrated that the Prophet (peace and blessings of Allaah be upon him) said: “The night prayers are two (rak’ahs) by two, then when you think that morning (dawn) is about to begin, pray one (rak’ah) and it will make what you have prayed Witr (odd-numbered).”
Shaykh Ibn ‘Uthaymeen said:
This indicates that the time for Witr ends when the dawn comes, because it is a prayer with which the night prayers are concluded, so it cannot be done after the night has ended.