Though several views are available on what exactly constitutes the time of birth, it would be worthwhile to safely assume that time at which the umblicle cord is cut , separating the mother from the child, may be taken as the time of birth. Care and precision should be undertaken in noting the exact time of birth as it is highly critical for all astrological predictions . It may be noted that there are several charts on which astrological predictions are based and an astrologer has to analyse and synthesize several charts before coming to a conclusion . In case your time of birth is approximate you should always let your astrologer know about it beforehand.
There is a birth chart and there are divisional charts focussing on specific aspects of life. The lagna or the ascendant of the birth chart may not change even for 90 minutes where as at other times you may be born at a lagna gandanta time or junction time where the lagna is changing say you may be born at times when ascendant or lagna sign may be changing. In such cases without establishing your correct time of birth it would be futile to go in with predictions as everything depends on the criticality of your date place and time of birth data. Astrology is mother of all sciences and highly precise.