Ekadasi fasting varies for Sri Vaishnavas, Madhwas and Smarthas.
If Dasami is present beyond 56 nazhigais (1 Nazhigai is 24 minutes), Ekadasi on the subsequent day is considered deficient; if Dasami exists even after 56 nazhigai, Srivaishanavas do not observe Ekadasi vratam on that day they observe vratam the following day.
For example, if on a Monday Dasami is present for more than 56 nazhigais, Ekadasi is not observed on the Tuesday. That is, because the Ekadasi tithi that occurs on a Tuesday is associated with Dasami during the brahma muhurtam on that Tuesday, Ekadasi is observed the next day on Wednesday.
The Smarthas consider Dwadasi as a basis and treat the day before Dwadasi as Ekadasi day, based on the details we find in a Panchangam.
As per panchangam, if it is smartha Ekadasi on the previous day, the subsequent day would be Srivaishanava Ekadasi.
As per Madhva tradition, if Dasami exists beyond 54 nazhigais, they consider it as tainted.
As can be seen from the panchangam, whenever Dasami is for more than 54 nazhigais but less 56 nazhigais, Srivaishnavas and Smarthas observe Ekadasi on the day before Madhva Ekadasi.