You know how the backs of seed packets often list "days to harvest?" Yeah, take those with a grain of salt. What I suspect they really mean is, "Days to harvest in precisely what we consider average conditions, even if there's no possible way you can reproduce our 'average' where you live."
I usually mark the projected "harvest day" on my calendar for non-fruiting items (carrots, lettuce, etc.) so I can check on them, but they're never accurate. Much of my lettuce could be harvested well before their date (but I let them keep growing!) and my carrots are nowhere big enough to harvest even though yesterday was the projected date. A million things effect the real harvest date. Weather, nitrogen levels, amount of sun, etc.
And for the record, many of the seed companies state that the days to harvest on the packets usually mean the number of days from the day you actually put the seeds or transplants out it in the garden. (See, isn't that silly? Of course a transplant will be ready to harvest sooner than a seed started on the exact same day!) This was one point of confusion for me when I started trying to read seed packets.
So, don't fret when a harvest date comes around and your carrots are still pencil thin (like mine!). Just use the "days to harvest" as a general indicator as to how fast-growing a particular variety is, or when it's almost time to start poking around the soil to see how big your root vegetables are.