Yields are calculated:

1. using a 30 day month ( as some banks do )
2. using a 360 day year ( as some banks do )

DomainYields.com uses an annualized interest calculation with a 360 day year, a 30 day month, a 24 hour day, a 60 minute hour, a 60 second minute and 0.1 or more nanoseconds.