Why is time depends on its …

latitude only specifies the north-south position of a point on earth's surface. But to calculate time, we need east-west position, as the earth rotates from west to east which can be specified by longitude. I believe, this is why the time of a place does not depends on its latitude.

Due to the rotation of the earth the time differed in different places. To standardize the time of different places of the earth and prevent confusion during long travels, the earth was divided into time zones. This was done by creating time zones that expanded to 15 degrees of longitude. The Greenwich Meridian was chosen as the standard time. The longitude of any place west of GMT has to be divided by 15 and the time would be that many hours ahead of GMT, and the ones in the east are that many hours behind GMT.