You don't _have_ to, I think previous commenter means that after a few days of feeling tired and groggy you will be tired earlier in the evening and go to bed earlier.
Adjusting in increments truly helps. If you usually sleep 8 hours, try waking half an hour early for a few days. It won't cause sleep deprivation, but you'll feel the need to go to bed early.
i think the idea isn't to sleep late and wake up early consistently. instead it's about waking up at the exact same time everyday, no matter how late you went to bed the previous night for reasons inside/outside your control. the expectation is that the next night you'll feel sleepy earlier and catch up.
in contrast, walking up late one day because you slept late the previous night will wreak havoc.
I'm only trying to clarify this because it works the same way for me.