Module:YMD to ISO

local p = {}

local function month_number(month_name)

   local months_full = {january=1, february=2, march=3, april=4, may=5, june=6, july=7, august=8, september=9, october=10, november=11, december=12}
   local months_abbr = {jan=1, feb=2, mar=3, apr=4, may=5, jun=6, jul=7, aug=8, sep=9, oct=10, nov=11, dec=12}
   local month_lc, _ = string.gsub(string.lower(month_name),'%.',,1)
   local month_num = months_full[month_lc] or months_abbr[month_lc] or 0
   if month_lc == 'sept' then
       month_num = 9
   end
   return month_num

end

local function days_in_month(month_num,year)

   -- modified from code in Module:Citation/CS1/Date_validation
   local days = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
   local month_length
   if month_num == 2 then         -- February: 28 days, unless leap year
       month_length = 28
       if year <= 1582 then       -- Julian calendar before Oct 1582
           if (year%4) == 0 then
               month_length = 29  -- if leap year, then 29 days
           end
       else                       -- Gregorian calendar since Oct 1582
           if ( (year%4)==0 and ((year%100)~=0 or (year%400)==0) ) then  
               month_length = 29  -- if leap year, then 29 days   
           end
       end
   else                           -- not February, get number of days for month
       month_length = days[month_num]
   end
   return month_length

end

local function zero_pad(string)

   if string.len(string) == 1 then
       return '0' .. string
   else
       return string
   end

end

function p.main(frame)

   if frame.args[1] == nil then
       return      -- first argument is missing
   end
   local arg1, _ = string.gsub(mw.text.trim(frame.args[1]),'_',' ')
   if arg1 ==  then
       return      -- first argument is empty
   end
   if not arg1:match('^%d%d%d%d %a%a%a%a?%.?%a?%a?%a?%a?%a?%a? *%d%d?$') then
       return arg1   -- invalid date pattern
   end
   local year, month_name, day = string.match(arg1, '^(%d%d%d%d) *(%a%a%a%a?%.?%a?%a?%a?%a?%a?%a?) *(%d%d?)$')
   if month_number(month_name) == 0 then
       return arg1   -- invalid month name or abbreviation
   end
   if tonumber(day) < 1 or tonumber(day) > days_in_month(month_number(month_name),tonumber(year)) then
       return arg1   -- invalid day number for given month
   end
   return year .. '-' .. zero_pad(tostring(month_number(month_name))) .. '-' .. zero_pad(day)

end

return p