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pythonpandasdatetimejulian-date
Stefano Potter
Stefano Potter

Reputation: 3577

Convert Gregorian date to Julian date

I have a df like so:

ImageDate   Year   Month   Day
20020506    2002   05      06
20030605    2003   06      05
20040201    2002   02      01

and I want to add a new column which converts the dates into julian dates. I am pretty sure I need the module datetime to do this, but I looked through the docs and didn't find anything that was apparent to me on how to do this.

Upvotes: 1

Views: 5651

Answers (3)

Prashy
Prashy

Reputation: 61

#Import python datetime library.
import datetime as dt
#Lets consider you want to convert the below date. You can have the  date in any order like CCYY-MM-YY or MM-DD-CCYY or with different separator like CCYY/MM/YY.
GregDate = '2020-03-30'
# Specify the date and format within strptime in this case format is %Y-%m-%d (if your data was 2020/03/30' then use %Y/%m/%d) and specify the format you want to convert into in strftime, in this case it is %Y%j. This gives out Julian date in the form CCYYDDD, if you need only in YYDDD give the format as %y%j
Juldate = dt.datetime.strptime(GregDate, "%Y-%m-%d").strftime('%Y%j')
# Print your variable to display the result
print(Juldate)

2020090

Upvotes: 0

mohd4482
mohd4482

Reputation: 1918

First Solution

The following is my python implementation of the original conversion algorithm published here:

def gregorian_to_julian(year, month, day):
    i = int((month - 14) / 12)
    jd = day - 32075
    jd += int((1461 * (year + 4800 + i)) / 4)
    jd += int((367 * (month - 2 - (12 * i))) / 12)
    jd -= int((3 * int((year + 4900 + i) / 100)) / 4)
    return jd

For example:

>>> gregorian_to_julian(1970, 1, 1)
2440588

Second Solution

alternatively, you can use the built-in date object to get the Julian date from Gregorian date by adding the Julian date of Gregorian absolute date 0 which is 1,721,425 to ordinal date as published here:

>>> from datetime import date
>>> date(1970, 1, 1).toordinal() + 1721425
2440588

The second solution is simpler and much faster.

Upvotes: 2

Jean-Michel Provencher
Jean-Michel Provencher

Reputation: 735

Go take a look at this library : https://pypi.python.org/pypi/jdcal

And do something like : `

from jdcal import gcal2jd, jd2gcal

gcal2jd(2000,1,1)'

Upvotes: 2

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