To calculate exact birthday one has to see length of left out part of the birth nakshtra. On or after sunrise if more than one tenth of the birth star is remaining it should be celebrated on that day else on the previous day. Like on an average 60 nazhikais are there for a nakshtra( some times it is shortened up to 55 only and on occasion is extended to more than 60. Please refer your Panchangam for exact span of that nakshtra) one tenth of it becomes 6 nazhikais. As such if it is more than 6 nazhikais on or after sunrise it should be celebrated on that day or else on previous day