If a Knight you control has lifelink, Syr Cadian won't cause you to gain additional life.

Knightlifelink is the same as granting lifelink to each Knight you control, but much, much cooler.

For determining which ability you can activate, use an official source that tracks when sunrise and sunset are in your area.

If it's the exact minute of sunrise or sunset, you may activate both abilities. But hurry!

Syr Cadian is a Knight, so knightlifelink applies to it.