Conversion to julian date of cycle dates

Cylc Cylc Support
1

Hi there. Our code mostly works with julian dates and starting from the cycle date, I need to set environment variables for start and end of each of each cycle in julian date format. What would be the best way to do this?
Cheers!
Gaby

2

Is this Julian dates as used in astronomy?

Using the formula from Wikipedia

(1461 × (Y + 4800 + (M − 14)/12))/4 +(367 × (M − 2 − 12 × ((M − 14)/12)))/12 − (3 × ((Y + 4900 + (M - 14)/12)/100))/4 + D − 32075

I’d say the best way to do this might be using a python script and the isodatetime library

Quick script to illustrate; made with assistance of GitHub Copilot; I’ve not tested this:

from metomi.isodatetime.datetimeoper import DateTimeOperator

cycle_point, _ = DateTimeOperator().date_parse('ref')  # uses $CYLC_TASK_CYCLE_POINT

julian_date = (
    1461 * (
        cycle_point.year + 4800 + (cycle_point.month_of_year - 14) // 12
    )
) // 4 + (
    367 * (
        cycle_point.month_of_year - 2 - 12 * ((cycle_point.month_of_year - 14) // 12)
    )
) // 12 - (
    3 * ((cycle_point.year + 4900 + (cycle_point.month_of_year - 14) // 12) // 100)
) // 4 + cycle_point.day_of_month - 32075
3

You could make a pull request to the open source Metomi-Isodatetime library used by Cylc to add a Julian Cycling mode. Effectively just some data inputting at isodatetime/metomi/isodatetime/data.py at master · metomi/isodatetime · GitHub

4

Oh and I guess I missed this out: you should be able to import the script in Jinja2 in the workflow in order to set the environment variables. See Jinja2 — Cylc 8.3.0 documentation

5

Ha, I will need to figure out which kind of Julian date they are taking about. Apparently there are different versions being used :dizzy_face:, but thanx for the tip about using a script

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