Writing time NetCDF file in python using NetCDF4

Question

I am trying to write a netcdf file using Netcdf4 library. My netcdf file has time, lat, lon and value variables. The time is monthly (i.e. 12 time values from 1/1/2000 to 12/1/2000).

I cannot write the time variable in the netcdf, my code is as follows:

import netCDF4 as nc
dsn = nc.Dataset(filename,'w', format='NETCDF4')
time = dsn.createDimension("time", None)
latd = dsn.createDimension("lat", len(lat))
lond = dsn.createDimension("lon", len(lon))

timev = dsn.createVariable("time", 'f4', ('time',))
latv = dsn.createVariable("lat", 'f4', ('lat',))
lonv = dsn.createVariable("lon", 'f 4', ('lon',))
value = dsn.createVariable("irr_req", 'f4', ('time', 'lat', 'lon',))
value.units = 'mm/month'

latv[:] = np.array(lat)  ## lat is an array with length 296 and lon witg 690
lonv[:] = np.array(lon)

## I am using relativedelta to write the monthly time data because I do not know another method-

from dateutil.relativedelta import * 

time =[datetime.datetime(2000, 1, 1) + relativedelta(months = month)
         for month in range(12)]

timev[:] = nc.date2num(time, "months since 2000-01-01 00:00" , calendar='360_day')

By doing this, I get the following error: RuntimeError: NetCDF: Not a valid ID

How can I write the time values? I am also ok with writing integers from 1 to 12 for months instead of the datetime variable.