Officially yes, you have to pay as of day 1 - well, more specifically, from the 1st of the month you arrive.
(I recently heard of someone who got here on December 31st, signed up as of January 1st and they made him pay for the whole of December even though he was only in the country for about 4 hours of the entire month!)
Yes, it is a strange one to get your head around but yes, you were insured for all of that time....
However, as I know a lot of companies (and more specifically, they know me!) I usually manage to wing it for people as long as we go for the "1st of" a month within 3 months