Observe the date on the following image for the Rancho San Emidio plat map:
January 1858 by Henry Hancock deputy Surveyor. Sound very plausible, and I took it for granted the date was correct. That is until I found Hancock's original survey notes.
Notice that the survey was not even begun until September of 1858. Hancock received his instructions on the first of the month and then commenced survey on the 18th. Last time I checked January does not come after September. That is a nine month difference.