Some babies will be tired enough to need to go to bed by 6:00 PM and if you need to put your baby to bed by 6:00 PM so that you can get ready to go to bed soon yourself then that's okay.
I have 5 kids and 2 of them when babies would fall asleep by 6:30 PM most nights so I would just put them to bed when they got tired enough to go to bed.
But all my babies were in bed by 7:00 PM whether they slept or not because I had to get ready for bed myself such as getting a shower, changing into pajamas, eating dinner etc.
Then I would have time to watch some TV and then get to bed.
If your baby is not tired by 6:00 PM then it might be too early for the baby to go to bed but if not then they can be put to bed by 6:00 PM