Finding when parents married in Modesto, California, USA

The FamilySearch wiki page for Stanislaus County, California genealogy lists the following sources for marriage records:

• 1800 - 2007 - California, United States Marriages at FindMyPast — index $
• 1850 - 1945 - California Marriages, 1850-1945 at FamilySearch — index
• 1850 - 1952 - California, County Marriages, 1850-1952 at FamilySearch — index and images
• 1854 - 1885 - Western States Marriage Index at BYU-Idaho
• 1960 - 1985 - California Marriage Index, 1960-1985 FamilySearch — index and images
• Early Stanislaus County Marriages at Genealogy Trails

FamilySearch is free to use, but you'll need to create a free account to view many of the records.

A cursory search of the webpages for Modesto and Stanislaus County brought me to the County Clerk-Recorder, where you can order copies of vital records. It doesn't give an e-mail addres, but a phone number is listed and this could be first step to confirm that they do in fact have your parent's marriage certificate.

I can't say for sure about the US or California, but generally these sorts of documents are indexed by name, so it should be possible for them to find the record even if you don't have the exact date.