This could help you
local dayValue, monthValue, yearValue = string.match('31-05-2014', '(%d%d)-(%d%d)-(%d%d%d%d)')
dayValue, monthValue, yearValue = tonumber(dayValue), tonumber(monthValue), tonumber(yearValue)
now = os.time{year = yearValue, month = monthValue, day = dayValue}
numberOfDays = now + 2 * 24 * 3600
print(os.date("%c",numberOfDays))
dateAfterNumberOfDays = os.date("%a %d %B %Y, %H%p%M",numberOfDays)
print ("\nafter number of days "..dateAfterNumberOfDays) -- give you date after number of days