Three elementary rules concerning time
1. When you refer to your work, you shall always and rigorously round up the remaining time.
Note that each started day counts as a finished day.
Example: On a Wednesday morning, you can say to your colleagues, "Only two days left!"
2. When you refer to your age before you're 18, you shall always round up the remaining years, months, days.
Note that anyway, you're much more mature than many of those gross grown-ups, aren't you?
Example: You're fifteen, and your mom tells you to be home at 11 p.m. First whine, "But Mom! I'm almost 18!" Then, confronted with her I-won't-budge-and-you-had-better-stop-it-right-now face, sulk.
3. However, when you refer to your age after you've celebrated your *cough-cough*-ieth birthday, you shall always and generously round off the years you've already lived.
Note that generous means like really big-time generous!
Example: You try on that hip jean, and when the vendor—who cannot even be almost eighteen! Aren't there any laws against child labour anymore?—only smirks, you tell him, "That's exactly what I need for my 29th birthday* celebration!"
* Note that you should adjust this number according to what the mirror REALLY tells you, though. Anything from 29 to 35 goes, as a rule.