Convert date to Unix time without os.time() - date

Question

I am developing a plugin written in Lua, and I need a way to calculate Unix time or at least a way to compare 2 date strings.
The function I can use only returns date string in the following format
"1/17/2014 6:50 PM"
Is there a way to convert this string to a Unix time?
Unfortunately I don't have access to the OS library so things like os.time() do not work.
Is there any library or something similar that I can work with?
I also thought about splitting the string into parts, but I need a way to add/subtract time

Answer 1

Just compare normalized timestamps:
function normalize(a)
local m,d,y,h,mi,n=a:match("(%d+)/(%d+)/(%d+)%s+(%d+):(%d+)%s+(%w+)")
if n=="PM" then h=h+12 end
return string.format("%04d%02d%02d%02d%02d",y,m,d,h,mi)
end
Date arithmetic is another story. For a complete, pure Lua date library, see luatz or https://github.com/Tieske/date.

Answer 2

If you need to only compare two time, you don't need to get each time's Unix timestamp. One possible solution is to get the time fields from the string like this:
local time = "1/17/2014 6:50 PM"
local month, day, year, hour, minute, am_pm = time:match("(%d+)/(%d+)/(%d+)%s+(%d+):(%d+)%s+(%w+)")
print(month, day, year, hour, minute, am_pm)
Output: 1 17 2014 6 50 PM
Then compare two time from comparing their year, if they are equal, then month, and so on. Remember to use tonumber to compare them by number, not the string itself.