Answer
Different systems of calculation are used by different traditions, especially in determining time according to the Vedic calendar.
The need for such calculations arises because time is measured in more than one way. For example, when we say today is a particular weekday or date, in the Vedic system the day is generally considered to begin at sunrise and end at the next sunrise. Thus, the transition from one day to the next is based on sunrise rather than midnight.
Similarly, observances such as Ekadashi are calculated according to the lunar calendar, which is based on the phases of the moon. The lunar month is divided into tithis, or lunar days, which do not always correspond exactly to the 24-hour solar day.
Because of this, a tithi may begin and end at any time within a solar day. Sometimes an Ekadashi tithi may begin before sunrise and end before the next sunrise, or overlap in different ways. This creates variations when compared with the standard solar calendar.
Therefore, different rules and systems have been developed by various acharyas to properly determine the observance times. As a result, there may be slight differences in calendars, but as long as one follows a bona fide, authorized system consistently, the observance is considered correct and spiritually valid.
The important point is to follow an authentic calendar and observe the rules properly with sincerity.
