Arunodaya
About 96 minutes before sunrise.
Four ghatis before sunrise. It is the moment at which the Ekadashi tithi is tested, and the reason an Ekadashi can be 'viddha' — contaminated by the preceding Dashami — and shifted.
What Arunodaya governs
These are tested at arunodaya, before dawn, which is why they can move to the following day when the preceding tithi is still running.