TBP clear it works like this;
ON Day 40 you can create a dot(s). It will have 8 days to train until the start of the next season. You can boost before end of current season so the dot will be level 4 at start of next season. Doing it this way gives you 2 mistake cushions... meaning you can miss 2 season's worth of boosts and you'll be given those 'missed boosts' at the end (Plateau).
You can create a dot(s) any day between Day 40 and Day 48 (before the season rolls over into next season) and you will be given whatever training points you missed by not creating ON Day 40. So if you created a dot on, say, Day 44 instead... then you would get 6 training points for the days that have already passed by (41, 42, & 43) to use just as if you had created on Day 40.
I'm not sure Novus is right (see post above) about creating a dot into the next season and getting TP's all the way back to last season but, TBH, I have not done that in... like... forever.
Even if you do, your dot will likely be 3 levels behind most dots they face throughout their careers until they hit Plateau because many boost early (the last day of the previous season). There's nothing wrong with this approach but it reduces your 'mistake cushion' down to only ONE season. If you miss 2 season's of boosts for any reason you will not be able to make up the difference when you reach plateau.