Problem with converting time
mikedoub at gmail.com
Fri Mar 17 16:20:03 EDT 2017
Sorry guys, I must not have been clear in my statement of the problem.
Here is the code:
put 1489755600 into tVar
convert tVar into dateitems
-- on the mac tVar contains 2017,3,17,9,0,0,6
-- on the server tVar contains 2017,3,17,12,0,0,6
The local time should not be involved since you are converting a fixed
value. What is happening?
On 3/17/17 12:44 PM, Richard Gaskin via use-livecode wrote:
> Michael Doub wrote:
> > time = 1489755600
> > Mac: dateitems = 2017,3,17,9,0,0,6
> > server: dateitems = 2017,3,17,12,0,0,6
> > Code: get time
> > convertit to dateitems
> > Richard here is another example:
> > time = 1489752000
> > mac: dateitems = 2017,3,17,8,0,0,6
> > server: dateitems = 2017,3,17,12,0,0,6
> > I just wanted to double check that I was showing you good comparison
> > since I am copying from multiple windows of debug output.
> > I don't understand how the local time comes into play when converting
> > a constant number of seconds. The resulting date should be a
> > constant as well.
> > What am I missing?
> The beauty of "the seconds" is that the value you get is in GMT,
> regardless of where the machine is at the moment that value of
> obtained. It does the offset according to the locale settings on the
> local machine.
> "The seconds" and "the internet date" are the only two forms that take
> local time zone settings into account.
> The benefit with this approach is that either can be used in
> applications where usage may span time zones but you need to maintain
> time stamps that will work across any time zones.
> For example, suppose I'm in Los Angeles and I share a server app with
> one user in New York, another in Brisbane, and another in Edinburgh.
> GMT offsets for each are:
> Edinburgh: 0
> New York: -4h
> Los Angeles: -7h
> Brisbane: +10h (they're across the International Dateline)
> The "internet date" format reflects this explicitly, noting the GMT
> offset at the end of the string it returns:
> Fri, 17 Mar 2017 09:09:03 -0700
> But "the seconds" of course is just an integer, so while the GMT
> offset has also been accounted for it's less evident.
> If all four of us get "the seconds" at the same moment, the value we
> get will be the same, in GMT time.
> But since only one of us is actually in the GMT time zone, how does it
> know? That's where the local system settings come into play.
> When *obtaining* either "the seconds" or "the internet date", local
> time zone is used to create a value that can be understood globally.
> When *converting* either "the seconds" or "the internet date", LC uses
> time conversion routines dependent on the system locale settings of
> the machine where the conversion is taking place.
> With "the internet date" the time zone is explicitly included, so it
> adjusts from that GMT offset to bring it into alignment with the GMT
> offset in the local OS.
> With "the seconds", the value is already adjusted to be GMT, so
> conversion adjusts it again for the local settings to deliver an
> accurate representation of that moment in local time.
> In the example above, all four users would get the same value from
> querying "the seconds" at the same moment, no matter where they are in
> the world.
> And when my software needs to display the date and/or time in
> human-readable form, the "convert" command takes local settings into
> account to deliver an accurate representation that makes sense in
> local time.
> So if I modify a record right now, in my system it'll show March 17 at
> 9:18 AM. But when my Brisbane use sees the modification date that
> I've stored using "the seconds", he'll see March 18 at 2:18 AM, which
> is the same global moment.
> In short:
> The key to getting accurate reflections of time is to store either
> "the seconds" or "the internet date" on the server (or really,
> anywhere timestamps may be shared across time zones), and then do any
> conversions needed for human readability locally.
> If you do a conversion from either of those two formats on the server
> to any other format, the result will be correct for the server's
> locale, but unless you're nearby probably not correct for you.
> Quick Exercise:
> It took me a while (and some experimentation) to appreciate that "the
> seconds" reflects GMT just as "the internet date" does, something I
> learned from Sarah Reichelt back in the day.
> One quick way to verify this is to check while changing your local
> machine's Date and Time settings:
> Set to Los Angeles: 1489768020
> 5 seconds later,
> Set to Montreal: 1489768025
> 10 seconds after that,
> Set to London: 1489768035
> Except for the few seconds it took me to click on my Locale map in my
> Control Panel, the numbers are effectively all the same.
> Once I became confident with that, I've used it for everything on
> servers ever since (except logging - I like the built-in readability
> of "internet date" in logs).
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