as salam alaikum
the time of Maghrib prayer starts at sunset. There is a disagreement among Shia scholars on when sunset starts. There are three major opinions in this regard:
1) Maghrib time start when the redness of the eastern sky disappears which is usually about 10-15 minutes after Sunni Maghrib time.
2) Maghrib time start at the disappearance of the sun in the western sky which is the same as Sunni Maghrib time.
3) Maghrib time starts at the disappearance of the sun in the western sky but as precaution we should wait until the redness in the eastern sky is disappeared.
The ahadith in support of the second opinion are accepted by all Islamic schools and are stronger in term of authenticity and numbers of isnads. However we find also ahadith stating the beginning of Maghrib time at the time of the disappearance of the redness in the eastern sky and that is why many scholars gave preference to that, at least as precautionary measure.
With prayers for your success.