--[[  __ __  _ _ ____ _  _   | \/ | ___ __| |_ _| | ___ _ / ___|__ _| | ___ _ __ __| | __ _ _ __   | |\/| |/ _ \ / _` | | | | |/ _ (_) | / _` | |/ _ \ '_ \ / _` |/ _` | '__|  | | | | (_) | (_| | |_| | | __/_| |__| (_| | | __/ | | | (_| | (_| | |   |_| |_|\___/ \__,_|\__,_|_|\___(_)\____\__,_|_|\___|_| |_|\__,_|\__,_|_|          Maintainers: * Jarekt ]]  local p = {}  -- Convert "Julian day number" (jdn) to a calendar date -- "gregorian" is a 1 for gregorian calendar and 0 for Julian -- based on https://en.wikipedia.org/wiki/Julian_day#Converting_Julian_or_Gregorian_calendar_date_to_Julian_day_number function p._jdn2date(jdn, gregorian)  local f, e, g, h, year, month, day   f = jdn + 1401  if gregorian>0 then  f = f + math.floor((math.floor((4*jdn + 274277) / 146097) * 3) / 4) - 38  end  e = 4*f + 3  g = math.floor(math.fmod(e, 1461) / 4)  h = 5*g + 2  day = math.floor(math.fmod (h,153) / 5) + 1  month = math.fmod (math.floor(h/153) + 2, 12) + 1  year = math.floor(e/1461) - 4716 + math.floor((14 - month) / 12)   -- If year is less than 1, subtract one to convert from a zero based date system to the  -- common era system in which the year -1 (1 B.C.E) is followed by year 1 (1 C.E.).  if year < 1 then  year = year - 1  end   return string.format('%04i-%02i-%02i', year, month, day) end  -- Convert calendar date to "Julian day number" (jdn)  -- "gregorian" is a 1 for gregorian calendar and 0 for Julian -- based on https://en.wikipedia.org/wiki/Julian_day#Converting_Julian_or_Gregorian_calendar_date_to_Julian_day_number -- explanation based on http://www.cs.utsa.edu/~cs1063/projects/Spring2011/Project1/project1.html function p._date2jdn(ISOdate, gregorian)   local year, month, day = ISOdate:match( "(-?%d%d%d%d)-(%d%d)-(%d%d)" )  if not year then  return nil  elseif tonumber(year) < 0 then  -- If year is less than 0, add one to convert from the common era system in which  -- the year -1 (1 B.C.E) is followed by year 1 (1 C.E.) to a zero based date system  year = year + 1  end  local a, b, c, d, y, m  a = math.floor((14-month) / 12) -- will be 1 for January and February, and 0 for other months.  y = year + 4800 - a  -- years since year –4800  m = month + 12*a - 3 -- month number where 10 for January, 11 for February, 0 for March, 1 for April  c = math.floor((153*m + 2)/5) -- number of days since March 1  if gregorian>0 then  b = math.floor(y/4) - math.floor(y/100) + math.floor(y/400) -- number of leap years since y==0 (year –4800)  d = 32045  -- offset so the result will be 0 for January 1, 4713 BCE  else  b = math.floor(y/4)  -- number of leap years since y==0 (year –4800)  d = 32083  -- offset so the result will be 0 for January 1, 4713 BCE  end  return day + c + 365*y + b - d end  -- Convert a date from Gregorian to Julian calendar function p.Gregorian2Julian(frame)  local JDN = p._date2jdn(frame.args[1], 1)  if JDN then  return p._jdn2date(JDN, 0)  else  return "Error parsing input date: " .. frame.args[1]  end end  -- Convert a date from Julian to Gregorian calendar function p.Julian2Gregorian(frame)  local JDN = p._date2jdn(frame.args[1], 0)  if JDN then  return p._jdn2date(JDN, 1)  else  return "Error parsing input date: " .. frame.args[1]  end end  -- Return day of week based on gregorian date. Mon->1, Tue->2, ..., Sun->7 function p.DayOfWeek(frame)  local JDN = p._date2jdn(frame.args[1], 1)  local day = math.fmod(JDN, 7) + 1  if day then  local LUT = { "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday" }  return LUT[day]  else  return "Error parsing input date: " .. frame.args[1]  end end  -- Convert calendar date to "Julian day number" (jdn) function p.date2jdn(frame)  return p._date2jdn(frame.args[1] or os.date('%F'), frame.args[2] or 1) end  return p