Is knowing the prayer timings essential for prayer. For example, if someone knows the time for prayer has begun but is unsure of whether or not the time for prayer has ended will his prayer be valid if he prays if he fears that the time might end if he tries to become sure.
All praise to Allah Swt and blessings on Prophet Muhammad peace be upon Him
Ibn Abbas narrated that the Prophet (PBUH) said, “Jibril (AS) led me in (in Salat) twice at the House (the Kaaba). So he prayed Zuhr the first time when the shadow was similar to (the length of) the strap of a sandal. Then he prayed Asr when everything was similar (to the length of) its shadow. Then he prayed Maghrib when the sun had set and the fasting person breaks fast. Then he prayed Isha when the twilight had vanished. Then he prayed Fajr when fajr (the dawn) began, and when eating is prohibited for the fasting person. The second time he prayed Zuhr when the shadow of everything was similar to (the length of) it, at the time of Asr the day before. Then he prayed Asr when the shadow of everything was about twice as long as it. Then he prayed Maghrib at the same time as he did the first time. Then he prayed Isha, the later one, when a third of the night had gone. Then he prayed Subh (Fajr) when the land glowed. Then Jibril (AS) turned towards me and said: O Muhammad (PBUH)! These are the times of the Prophets before you, and the time is what is between these two times.”(Abu Dawud)
by which one can understand praying on time is essential
Ibn Umar narrated that : Allah's Messenger said: "The beginning of the time for Salat is pleasing to Allah, and the end of its time is pardoned by Allah."(At Tirmidhi)
In Modern era there are lots of ways to figure out the timings of prayers unlike earlier era we are not only dependant on shadows and stars,lots of applications in devices ,some softwares ,these could help us in identifying correct prayer timings however if someone is in a situation when he is not aware of the time and he prays just by his own sense visualizing Sun or darkness/brightness and this was not his routine like may be one time or second so Allah Swt is Raheem could be acceptable
Ali bin AbI Talib narrated that : the Prophet said to him:
'Ali! Three are not to be delayed: Salat when its time comes, a funeral whet it (a prepared body) is present, and the (marriage of a) single woman when there is an equal for her."(At Tirmidhi)