Different scholars have different views on this issue.
Sahar time is seconds before Fajr.
Fajr is the horizontal light that emerges out of the darkness. Before that is a vertical light which is called the False Fajr.
Zuhr is when the shadow of the sun disappears when you have a stick. Sometimes it may not totally disappear depending on where you are, however it is the moment it reaches its lowest and starts growing after.
Asr is about 4 minutes after Zuhr time. The first 4 minutes are dedicated for Zuhr only.
Maghrib is when the redness in the eastern horizon disappears.
Ishaa is about 3 minutes after maghrib. The first 3 minutes are exclusively for maghrib.
Ishaa prayer becomes Qazaa with midnight, which is the point between sunrise and sunset.
The midnight according to sayed Sistani is between fajr and sunset
While sayed Khoie says it is between sunrise and sunset