It maybe Myth, but, when you stop to think about it, every big quake to comes to my mind has hit in the early morning or late evening and NEVER in the mid day.

Landers-early AM
Big Bear-early AM
Northridge-early AM
Hector-Early AM
Whittier-Early Am
Big Bear quake of 3 years ago-early AM
1971 San Fernando-Early AM
1952 Kern County Quake-Early AM
1906 Early AM
1857 AM
1872 Owens Valley-Early AM

Then there is a splattering of moderate Bay ARea quakes that occured in early AM, 1986 Mt. Lewis, Yountville, Los Gatos in 1989.

The next tier are all before noon-San Simeon, Parkfield and Long Beach and Loma Prieta fall into the late afternoon evening. Not too much between like 12 noon and 4pm.