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DAYMS = 1000 * 60 * 60 * 24

checkBirthday = ( now, bd, diff )->
  # differ between next and last days (look back or forward)
	if diff > 0
		start = new Date( now + " 00:00:00 +0000" )
		end = new Date( start.valueOf() + ( DAYMS * diff ) )
		year = new Date( end ).getFullYear()
	else
		end = new Date( now + " 00:00:00 +0000" )
		start = new Date( end.valueOf() + ( DAYMS * diff ) )
		year = new Date( start ).getFullYear()

  # reset the year to be comparable 
	birthday  = new Date( bd + " 00:00:00 +0000" ).setFullYear( year )
	console.log( 'birthday', start, " : ", new Date( birthday ), " : ", end )
  
  # check if the changed birthday is within the start and end date
	if birthday >= start and birthday <= end
		return true
	else
		return false

# Test the edge cases
# now, birthday, diff, check
cases = [
	[ "A", "2017-01-02", "1985-01-02", 7, true ]
	[ "B1", "2017-01-02", "1985-01-09", 7, true ]
	[ "B1", "2017-01-01", "1985-01-09", 7, false ]
	[ "B2", "2017-01-02", "1985-01-08", 7, true ]
	[ "C", "2016-12-31", "1985-01-01", 7, true ]
	[ "D", "2017-01-01", "1985-12-31", -7, true ]
	[ "E1", "2016-02-22", "2000-02-29", 7, true ]
	[ "E2", "2016-02-22", "2001-02-29", 7, false ]
	[ "F", "2015-02-22", "2000-02-29", 7, true ]
	[ "G", "2016-12-28", "1985-01-02", 7, true ]
	[ "H", "2016-12-28", "1985-01-02", -7, false ]
	[ "I", "2016-12-28", "1985-01-02", -7, false ]
	[ "J", "2016-01-04", "1985-12-29", -7, true ]
	[ "K", "2016-01-04", "1985-12-29", -7, true ]
]

for [ name, now, bd, diff, res ] in cases
	if checkBirthday( now, bd, diff ) is res
		console.log(name, "OK" )
	else
		console.log(name, "Failed" )