nc_time_dim_update.py

import netCDF4
from datetime import datetime
import pytz
import numpy as np

with netCDF4.Dataset("a_FMC_File.nc", 'r+', format='NETCDF4') as src:
    temp = np.zeros(src["time"].shape)
    for i, t in enumerate(src["time"]):
        d_local = datetime.fromtimestamp(t)
        t_utc = (pytz.utc.localize(d_local)-pytz.utc.localize(datetime(1970,1,1))).total_seconds()
        temp[i] = t_utc
        
    src["time"][:] = temp