Vars within function are not reset per call (LUA) - function

Question

I am trying to set a function to add min to current time object and return a new time object. I created a function for this but for some reason the vars in the functions are not been re-set / local each time I call the function.
each call to the function will use the past value of the local vars within the function, why ?
local function AddTime (MinAfter, BaseTime)
if (MinAfter == nil) then MinAfter = 0 end
if (BaseTime == nil) or (BaseTime.min == nil) or (BaseTime.hour == nil) then BaseTime = os.date("*t") end
BaseTime.hour = BaseTime.hour + math.floor((BaseTime.min + MinAfter)/60)
BaseTime.min = BaseTime.min + MinAfter - (60 * (math.floor((BaseTime.min + MinAfter)/60)))
if BaseTime.hour > 24 then BaseTime.hour = 24 end
return BaseTime
end
local sunriseHour = os.date("*t" ,os.time {year = 2014, month = 4, day = 19, yday = 259, wday = 4, hour = 6, min = 0, sec = 0, isdst = false});
-- this is the original time object in this case sunraiseHour
print ("sunriseHour time:" .. (string.format("%02d",sunriseHour.hour) .. ":" .. string.format("%02d", sunriseHour.min)));
-- first call
local newtime1= AddTime(10, sunriseHour);
print ("call 1 time:" .. string.format("%02d", newtime1.hour) .. ":" .. string.format("%02d", newtime1.min));
-- on the 1st call I get 07:10 which is right
-- 2nd call
local newtime2= AddTime(10, sunriseHour);
print ("call 1 time:" .. string.format("%02d", newtime2.hour) .. ":" .. string.format("%02d", newtime2.min));
-- on the 2nd call I get 07:20 and not 07:10 since this was the 2nd call to the function - the BaseTime var within the function was not local

Answer 1

When you pass sunriseHour into AddTime, it is passed by reference rather than by value, which means that any changes made to BaseTime inside of AddTime are changes to sunriseHour -- both varibles (sunriseHour and BaseTime) point to the same object.
So when you write the following in AddTime:
BaseTime.hour = BaseTime.hour + math.floor((BaseTime.min + MinAfter)/60)
BaseTime.min = BaseTime.min + MinAfter - (60 * (math.floor((BaseTime.min + MinAfter)/60)))
You're modifying sunriseHour.
It seems you don't quite understand this, because you also assign a new value to BaseTime inside of AddTime, which suggests you think you have a new object. If you want to create an altered copy of sunriseHour, then you'll need to either do that inside of AddTime, or create some kind of copy constructor for your time object.

Answer 2

thanks Mud,
I understood that I needed to copy my time object otherwise since I use a reference of it, it will modify the original object.
I found a copy function and use it to copy the object into a local one within the function
thanks
function table.copy(orig)
local orig_type = type(orig)
local copy
if orig_type == 'table' then
copy = {}
for orig_key, orig_value in pairs(orig) do
copy[orig_key] = orig_value
end
else -- number, string, boolean, etc
copy = orig
end
return copy
end
function AddTime (MinAdd, TimeObj)
local BaseTime = {};
local MinAfter = 0;
if (TimeObj == nil) or (TimeObj.min == nil) or (TimeObj.hour == nil) then BaseTime = table.copy(os.date("*t")) else BaseTime = table.copy(TimeObj) end;
if (MinAdd == nil) then MinAfter = 0 else MinAfter = MinAdd end;
BaseTime.hour = BaseTime.hour + math.floor((BaseTime.min + MinAfter)/60)
BaseTime.min = BaseTime.min + MinAfter - (60 * (math.floor((BaseTime.min + MinAfter)/60)))
if BaseTime.hour > 24 then BaseTime.hour = 24 end
return BaseTime
end
-- this is the original time object in this case sunraiseHour
local sunriseHour = os.date("*t" ,os.time {year = 2014, month = 4, day = 19, yday = 259, wday = 4, hour = 6, min = 0, sec = 0, isdst = false});
print ("sunriseHour time:" .. (string.format("%02d",sunriseHour.hour) .. ":" .. string.format("%02d", sunriseHour.min)));
-- first call
local newtime1= AddTime(10,sunriseHour);
print ("call 1 time:" .. string.format("%02d", newtime1.hour) .. ":" .. string.format("%02d", newtime1.min));
-- on the 1st call I get 07:10 which is right
-- 2nd call
local newtime2= AddTime(10,sunriseHour);
print ("call 2 time:" .. string.format("%02d", newtime2.hour) .. ":" .. string.format("%02d", newtime2.min));
-- on the 2nd call I get 07:20 and not 07:10 since this was the 2nd call to the function - the BaseTime var within the function become global
print ("Added time:" .. string.format("%02d", AddTime(20, sunriseHour).hour) .. ":" .. string.format("%02d", AddTime(20, sunriseHour).min));