Doesn’t account for region
The moon phases in different parts of the world are different. For eg. Next Ekadasi in my region (Northern California) is January 31st . But the calendar still shows Indian calendar date (Feb 1st) . Next full moon here is Feb 5th , but calendar shows me Feb 6th (full moon date in india). That’s just bad calculation. We need to start our fasts and meditations at correct times in our current locations.