This Swiss government document explains when compulsory health insurance is to start in Switzerland. If you are registered, it will apparently be backdated to your date of registration, according to the below:
"06. Within which period do I have to take out insurance, and what are the consequences of delayed registration?
You have to take out health insurance within three months of the beginning of compulsory insurance (e.g. of the date you take up residence or give birth to a child in Switzerland). The insurance is then back-dated to the beginning of compulsory insurance. Since expenses are reimbursed retroactively, consequently it is mandatory to pay the premiums retroactively from the beginning of coverage on.
If you do not meet this three-month deadline, your insurance cover will only take effect from the date you register, and you will have to pay a premium supplement unless you can give good reasons for the delay." http://www.bag.admin.ch/themen/krank...x.html?lang=en