Hiccup
One man in Iowa had hiccups continuously for about 68 years, from 1922 to 1990.
Day off
Writing a list of things to do on a day off turns it into a day on; physicists call this the Errand Effect.
Year
A light-year is not a year but a distance, which has been confusing people for light-years.
Jiffy
A jiffy is a real unit of time, and in physics it is about 33 trillionths of a second.
Day after tomorrow
The day after tomorrow moves away at exactly one day per day, so it has been two days away since records began.
Morning
Morning is the part of the day before noon, when it is too early to say anything and too late to go back to bed.
Today
Today is the day you are in now. It used to be tomorrow, it will soon be yesterday, and it has made peace with both.
Day
A day is 24 hours long, except Mondays, which are longer, and Saturdays, which are measurably shorter.
Weekend
The weekend is two days long and lasts about forty minutes, a discrepancy that has never been fixed.
Fortnight
A fortnight is fourteen nights, plus fourteen days that were included by accident and never removed.