Why does one have to break Ekadashi at a specific time?
Brief explanation of the timing by H G Bhaktarupa Pr:
The basic rule is that one should wait until after sunrise to break the fast. Sunrise times change from day to day, thus the beginning of the parana time varies. And one should not wait forever on the next day to break. The tradition is that under normal circumstances one should break ekadasi during the first one-third of the daylight hours on that day. Thus the ending of the parana time varies. Then there are sometimes exceptions. The ekadasi tithi plus 25% of the dvadasi tithi is called Hari Vasara. One should not break fast until after that period is over, but before the end of the dvadasi tithi. Thus sometimes one has to wait until some time after sunrise to break, and sometimes one has to break quickly after sunrise. There are other exceptions also. Like we just had a Vyanjuli Mahadvadasi. After this mahadvadasi the break fast time period is always short because the dvadasi tithi ends soon after sunrise.