Pandas datetime with Julian Day

Question

I tried searching for this and was surprised I couldn\'t find anything. We use the term \'Julian Day\' to refer to the day of the year irrespective of month (i.e. February

Answer 1

When you read in the Julian date file, you simply need to provide a custom date parsing function. Here's an examples:

import datetime
from io import StringIO
import pandas

datafile = StringIO("""\
jday,value
2013-01,1
2013-02,2
2013-100,8
2013-200,9
""")

dateparser = lambda x: datetime.datetime.strptime(x, '%Y-%j')
df = pandas.read_csv(datafile, parse_dates=True, date_parser=dateparser, index_col=[0])

Which gives a df of:

            value
jday             
2013-01-01      1
2013-01-02      2
2013-04-10      8
2013-07-19      9

I keep this page bookmarked and handy for "unconventional" date parsing needs such as these. (I don't actually think julian days are weird -- we use them all the time in hydraulic modeling)

Answer 2

Try dayofyear. Julian day is actually a completely different number FYI, see here

In [1]: pd.date_range('20130201',periods=5).dayofyear
Out[1]: array([32, 33, 34, 35, 36], dtype=int32)