Python: convert 'days since 1990' to datetime object

Question

I have a time series that I have pulled from a netCDF file and I'm trying to convert them to a datetime format. The format of the time series is in 'days since 1990-01-01 00:00:00 +10' (+10 being GMT: +10)

time = nc_data.variables['time'][:]
time_idx = 0  # first timestamp 
print time[time_idx]

9465.0

My desired output is a datetime object like so (also GMT +10):

2015-12-01 00:00:00

I have tried converting this using the time module without much success although I believe I may be using wrong (I'm still a novice in python and programming).

import time
time_datetime = time.strftime('%Y-%m-%d %H:%M:%S', time.gmtime(time[time_idx]*24*60*60))

Any advice appreciated, Cheers!

Answer 1

The datetime module's timedelta is probably what you're looking for.

For example:

from datetime import date, timedelta

days = 9465                 # This may work for floats in general, but using integers
                            #   is more precise (e.g. days = int(9465.0))

start = date(1990,1,1)      # This is the "days since" part

delta = timedelta(days)     # Create a time delta object from the number of days

offset = start + delta      # Add the specified number of days to 1990

print(offset)               # >>>  2015-12-01
print(type(offset))         # >>>  <class 'datetime.date'>

You can then use and/or manipulate the offset object, or convert it to a string representation however you see fit.

You can use the same format as for this date object as you do for your time_datetime:

print(offset.strftime('%Y-%m-%d %H:%M:%S'))

Output:

2015-12-01 00:00:00

---

Instead of using a date object, you could use a datetime object instead if, for example, you were later going to add hours/minutes/seconds/timezone offsets to it.

The code would stay the same as above with the exception of two lines:

# Here, you're importing datetime instead of date
from datetime import datetime, timedelta

# Here, you're creating a datetime object instead of a date object
start = datetime(1990,1,1)   # This is the "days since" part

Note: Although you don't state it, but the other answer suggests you might be looking for timezone aware datetimes. If that's the case, dateutil is the way to go in Python 2 as the other answer suggests. In Python 3, you'd want to use the datetime module's tzinfo.

Answer 2

netCDF num2date is the correct function to use here:

import netCDF4

ncfile = netCDF4.Dataset('./foo.nc', 'r')
time = ncfile.variables['time'] # do not cast to numpy array yet 
time_convert = netCDF4.num2date(time[:], time.units, time.calendar)

This will convert number of days since 1900-01-01 (i.e. the units of time) to python datetime objects. If time does not have a calendar attribute, you'll need to specify the calendar, or use the default of standard.