[erlang-questions] calendar now_to_universal_time/1 vs. universal_time/0

[Alexander Harju]

On Mon, Sep 27, 2010 at 9:15 PM, Ryan Zezeski <rzezeski@REDACTED> wrote:

> On Mon, Sep 27, 2010 at 1:00 PM, Ulf Wiger
> <ulf.wiger@REDACTED>wrote:
>
> >
> > On 27 Sep 2010, at 18:25, Pierpaolo Bernardi wrote:
> >
> > > On Mon, Sep 27, 2010 at 18:21, Ryan Zezeski <rzezeski@REDACTED>
> wrote:
> > >
> > >> This is, essentially, the question I'm asking.  Should I prefer one
> over
> > the
> > >> other for *any* reason?  Right now, I prefer universal_time because
> it's
> > >> less typing :)
> > >
> > > A perfectly good reason.  It's also a tiny bit clearer.
> >
> > erlang:now() is actually a bit different from the calendar clock.
> > It can, by definition, never jump backwards.
> >
> > It will also attempt not to make large adjustments of any kind, so if it
> > detects
> > that there is a large difference between the system clock and the
> > erlang:now()
> > clock, the now() clock will speed up or slow down 1% in order to converge
> > with the actual time, without disturbing timeouts etc. which rely on
> now()
> > to
> > provide a smooth representation of system real-time.
> >
> > In other words, erlang:universal_time/0 will return the actual time,
> > whereas
> > erlang:now() may, under certain circumstances, differ quite significantly
> > from actual time.
> >
> >
> Thanks Ulf, but I was actually referring to calendar:universal_time/0.  I
> did not know of the existence of erlang:universtaltime/0 until you pointed
> it out just now.  Now I'm curious, what is the difference, if any, between
> these two?  Is their existence just a case of history?
>
> Another question, both functions claim that they return UTC "if available"
> otherwise they return local time.  In what case is UTC not available?
>

calendar:universal_time() calls erlang:universaltime() so the difference is
just another function call.

Regarding now().
You shouldn't call calendar:now_to_universaltime(now()). That doesn't make
any sense at all.
now() has to be unique, therefore it's MUCH slower.
The only time it make sense to to use calendar:now_to_universaltime(...) is
when you have a now-timestamp and want to convert it to universal-time.

If you need a timestamp higher precision than seconds I recommend to write
your own now-nif to get rid of the penalty of now().

// Alex


>
> -Ryan
>