Here's the definition from FTB:
The base year means taxable year 2013 for a qualified full-time employee hired during taxable year 2014 or if a qualified taxpayer first hires a qualified full-time employee in a taxable year beginning on or after January 1, 2015, it is the taxable year immediately preceding the taxable year in which the qualified full-time employee was hired.
For taxpayers who first commence doing business in this state during the taxable year, the number of full-time employees for the base year shall be zero.
So for example if an employee was hired in 2017, the base year would be 2016.
