The timezone database (maintained by the people who are very particular about making sure that a time specified is a well known time) have a note in the northamerica data file:

https://github.com/tzinfo/tzinfo-data/blob/master/data/north...

# From Paul Eggert (2016-08-20):

# In early February 1948, in response to California's electricity shortage,

# PG&E changed power frequency from 60 to 59.5 Hz during daylight hours,

# causing electric clocks to lose six minutes per day. (This did not change

# legal time, and is not part of the data here.) See:

# Ross SA. An energy crisis from the past: Northern California in 1948.

# Working Paper No. 8, Institute of Governmental Studiåes, UC Berkeley,

# 1973-11. http://escholarship.org/uc/item/8x22k30c