Week Number In Year

wiki

Normally you can call

print("W"..os.date( "%V"))

but in my Lua 5.1 that was not working. So I wrote my code using

os.date("*t",tm) -- to get table

os.date("%j",tm) -- to get year day

os.date("%w",tm) -- to get week day

-- Get day of a week at year beginning 
--(tm can be any date and will be forced to 1st of january same year)
-- return 1=mon 7=sun
function getYearBeginDayOfWeek(tm)
  yearBegin = os.time{year=os.date("*t",tm).year,month=1,day=1}
  yearBeginDayOfWeek = tonumber(os.date("%w",yearBegin))
  -- sunday correct from 0 -> 7
  if(yearBeginDayOfWeek == 0) then yearBeginDayOfWeek = 7 end
  return yearBeginDayOfWeek
end

-- tm: date (as retruned fro os.time)
-- returns basic correction to be add for counting number of week
--  weekNum = math.floor((dayOfYear + returnedNumber) / 7) + 1 
-- (does not consider correctin at begin and end of year) 
function getDayAdd(tm)
  yearBeginDayOfWeek = getYearBeginDayOfWeek(tm)
  if(yearBeginDayOfWeek < 5 ) then
    -- first day is week 1
    dayAdd = (yearBeginDayOfWeek - 2)
  else 
    -- first day is week 52 or 53
    dayAdd = (yearBeginDayOfWeek - 9)
  end  
  return dayAdd
end
-- tm is date as returned from os.time()
-- return week number in year based on ISO8601 
-- (week with 1st thursday since Jan 1st (including) is considered as Week 1)
-- (if Jan 1st is Fri,Sat,Sun then it is part of week number from last year -> 52 or 53)
function getWeekNumberOfYear(tm)
  dayOfYear = os.date("%j",tm)
  dayAdd = getDayAdd(tm)
  dayOfYearCorrected = dayOfYear + dayAdd
  if(dayOfYearCorrected < 0) then
    -- week of last year - decide if 52 or 53
    lastYearBegin = os.time{year=os.date("*t",tm).year-1,month=1,day=1}
    lastYearEnd = os.time{year=os.date("*t",tm).year-1,month=12,day=31}
    dayAdd = getDayAdd(lastYearBegin)
    dayOfYear = dayOfYear + os.date("%j",lastYearEnd)
    dayOfYearCorrected = dayOfYear + dayAdd
  end  
  weekNum = math.floor((dayOfYearCorrected) / 7) + 1
  if( (dayOfYearCorrected > 0) and weekNum == 53) then
    -- check if it is not considered as part of week 1 of next year
    nextYearBegin = os.time{year=os.date("*t",tm).year+1,month=1,day=1}
    yearBeginDayOfWeek = getYearBeginDayOfWeek(nextYearBegin)
    if(yearBeginDayOfWeek < 5 ) then
      weekNum = 1
    end  
  end  
  return weekNum
end  

useDate = os.time{year=2015,month=9,day=30}
weekNum = getWeekNumberOfYear(useDate)
str = os.date("%Y/%m/%d",useDate)..": week number:"..tostring(weekNum)
print(str)

It was cross checked with following python code:

import datetime
dt = datetime.date(1970, 1, 1)
dtAdd = datetime.timedelta(hours=24)

f = open('pyDate.txt','w')
for i in range(0,365*200+1):
  txt = str(dt.strftime("%Y/%m/%d"))+":W"+str(dt.isocalendar()[1])  
  #print(txt)
  f.write(txt+"\n")
  dt += dtAdd
f.close()