Some online databases like to tease users with titles that indicate the database is more encompassing than it actually is. Make certain you know what a database contains before searching it. The title may indicate that the marriage records are from 1800-1920, but there may be a twenty year gap of missing records in the mid-1800s.
Of course, that will be the twenty year time span you actually need.
Check out my 1950 census preparation webinar-1 April is coming up!