Legal Question in Civil Litigation in California

Neither.

Unless you specifically agreed to accept service by email, or the court ordered it, email is not an approved method for serving any papers in your case. (See Code of Civil Procedure Section 1010.6.) Thus, you should rely on the mailing date, not the date your received them. You have 30 days from the date of the mailing, which should be specifically stated on a Proof of Service that the other party should have included with the interrogatories. In fact, you have five extra calendar days if the interrogatories were served by mail, for a total of 35 days.

If the interrogatories were sent by overnight or express mail, you get an extra two court days. (See Code of Civil Procedure Section 1013.)

I'm sure you have heard this already (especially if you are a defendant in an unlimited civil case), but you really should look into doing whatever it takes to hire an attorney. There are so many rules you won't know about that could cause problems for you. The courts won't be sympathetic to you not having an attorney because pro pers are held to the same standards as attorneys.

Good luck!