Technique to delete spin-up frames from WRF
Here is my  present approach.    


All wrfout-files are located in the same path.
Using pynco in python loop to split the spin-up dataframe

import glob
nco = Nco()
for fn in glob.iglob('*.nc'):
    filename, extname = os.path.splitext(fn)
    output_fn = filename[11:22] + str(int(filename[22:24]) + 12) + extname
    nco.ncks(input=fn, output=output_fn, dimension='Time,13,')  



The input shows like: 2014-04-04_12.nc(120 frame inside)
The output shows like:wrfout_2014-04-04_00:00:00.nc(133 frame inside)