I am working on a database of historical figures and events. I have two custom post types: 'figures' and 'events'. The plan is when reading a post in 'figures', the theme would compare the figure's birth (start) and death (end) dates with posts in 'events', and any events post between those two dates would display on the figure's post. This part I know how to do.
Some dates are pre-100AD, so each 'figures' start and end dates should be a Julian Day (JD). This would be fed from three custom fields each: start_year, start_month, and start_day for start, and end_year, end_month and end_day for end.
Rather than constantly convert each event into a JD with a separate converter and enter the result manually, I would prefer that the information from each of these fields is automatically fed into either the custom fields start_jd or end_jd, dependning.
Any date before the first day of the Gregorian calendar would use juliantojd() and any date on or after would use gregoriantojd().
So if I enter a start date of:
start_year = 756
start_month = 12
start_day = 31
...hitting "publish" would automatically fill 'start_jd' with the JD '1997552'. Likewise, if I enter an end date of:
end_year = 1870
end_month = 9
end_day = 20
...hitting "publish" would automatically fill 'end_jd' with JD '2404326'.
tl;dr:
How do/can I get WordPress to automatically convert a Julian or Gregorian calendar date into a JD, and then automatically write that JD into a custom field where it is saved?
Able to get up to filling out the y, m and d custom fields, and know how to use php's gregoriantojd() and juliantojd() functions.