time_difftime.lua

local day1 = { year = 2015, month = 7, day = 30 }
local t1 = os.time(day1)

local day2 = { year = 2015, month = 7, day = 31 }
local t2 = os.time(day2)

print(os.difftime(t2, t1))

-- output
-- 86400