Hey Growmie,
Most estimated harvest dates are usually wrong so I wouldn't go off that. It is best to take a close look at the trichomes on your plant to assess when your cannabis is likely to be ready for harvest. If the tiny trichomes are just beginning to turn from clear, to a cloudy and milky color, this could be a good indication that the plant can begin flushing. It should be timed so that the majority of trichomes will have fully changed to the desired colour for harvest after two weeks.
Hopefully, I answered your question and remember "Happy Growing" and "Free your mind one puff at a time"