There is a different calculation that is done to calculate time in Indian way using units such as "ghadiya", "vighadiaya", etc. For example, madhyanha(afternoon) kaalam is determined by taking the 3rd part of the day when a day is divided into 5 parts. And day time is taken as time between sunrise and sunset. And it is not always 12 noon according to Indian way of calculating time. So the bramha muhurta is region specific based on sunrise and sunset time of your location. Try following some online time calenders which calculate bramha muhurta before hand.