Do Tenants need to give 30 day notice (written or not) to move out?

[Jorge Liang]

Received an email from the tenants that they are moving out at the end of the month on 1/15. 

Are CA tenants required to give a 30-day notice? If so, can I hold the tenants accountable for the 30 days after the day I receive the email? Does the notice have to be via certified mail or will Email be sufficient? What does the law say? 

Per Nolo - "Unless the rental agreement provides a shorter notice period, a California tenant must give their landlord 30 days' notice to end a month-to-month tenancy".  However, I can't find the specific CA policy to verify this.