Re: how to compare 2 dates - NOW WITH TIME

[KHMan <keinhong@...>]

Diego - Red Baires wrote:
> [snip]
> premise:
> i need to represent a range or dates, and then a range of time.
> i have a list of records that indicates that an event must be
> executed only if:
> a) today is between the fields that indicates a range of dates, and then
> b) os system.clock is between the fields that indicates a range of hours.
>
> (is today between date-A and date-B, if so, is now between 3pm and 4pm?)
>
> so i am doing this:
>
> -----------
> between_date_from = os.time({ year="2008", month="01", day="01", 
> hour="00", min="00" })
> between_date_to   = os.time({ year="2009", month="05", day="06", 
> hour="00", min="00" })

On many platforms, this os.time() function which calls the 
platform's equivalent will happily accept 'illegal' values like 
month=13 or day=40. You'd get sorta a 'best guess' value. (Note: 
You supply numbers; strings like "2009" are turned into numbers 
like 2009. You might also not need to supply zero fields like 
min=0, because the function does a best guess.)

Thus validating a date is non-trivial. Also, depending on user 
input, dates are of various precisions. More validation, if your 
application expects certain formats or precisions. Then to be 
precisely precise, you'd also need to accept sec=60 for some 
years, which of course, I think most OS don't deal with a leap 
second in the OS layer. Then there is nntp synchronization. And so on.

> [snip snip]
> now the problem is how to analize a range of hours, not taking in mind 
> the date.
> any idea ?
>
> "os.time()" does not allow (obviously) an assignation like this:
> { year="0000", month="00", day="00", hour="17", min="30" }

os.time() on most platforms (POSIX-ish) is the number of seconds 
after the Unix epoch. You can trivially compare and subtract the 
times. The difference is in seconds, which you can then convert 
into anything you like.

If you are in total control of an application, and need a standard 
textual format for date/time, then use ISO 8601 [1](or a subset), 
and make your users use it. Make your program validate ISO 8601 
time formats. Once it validates correctly, common ISO 8601 formats 
like YYYY-MM-DD can also be compared trivially in string form. 
Using the ISO form also avoids ambiguity common with date/time 
formats of different countries. More info in [1], link below.

[1] http://en.wikipedia.org/wiki/ISO_8601

--
Cheers,
Kein-Hong Man (esq.)
Kuala Lumpur, Malaysia