The expression is not necessarily true. 2/7 of a week = 2 days IS true. But 2/7 of a week = 2 (any time period other than a day) is a false statement.
A fraction can be considered in this way: The denominator is the number of pieces into which the whole is divided, and the numerator is the number of those pieces you have. So if the time period above is in fact days, then there are 7 days in a week and that is the source of the denominator. So if you divide a week into 7 equal pieces, each one is a day, and 2 of those pieces make 2 days.
My calculator said it, I believe it, that settles it