Implicit local timezone assumed in Epoch about dates.jl

[quinnj]

@aviks, let me stress again, the confusion/problem is not in datetime2unix/unix2datetime, which are very straightforward methods. Let's breakdown what everything is doing here:

• unix2datetime takes the # of seconds since 1970-01-01T00:00:00 GMT and converts to a DateTime
• datetime2unix takes a Julia DateTime and converts it to the # of seconds since 1970-01-01T00:00:00 GMT, with an implicit assumption that the DateTime instance is GMT
• now(Dates.UTC) returns the current wall-time as a DateTime in GMT
• now() returns the current wall-time as a DateTime adjusted according to the local system locale

I think your confusion comes from the fact that now() is returning a DateTime that will show exactly what I see in the corner screen of my system, yet that DateTime type is inherently non-timezone aware, so passing that value to datetime2unix falls prey to the "implicit assumption that the DateTime is GMT".

We really can't do any better with the current, minimal Base implementation. We could change now() to return the current wall-time in GMT and have a separate now(Dates.LOCAL) option for current wall-time according to system time, but that's 6s to me vs. the current setup.

My point about Java being timezone aware is the fact that when you ask for the current date, they're implicitly doing now(Dates.UTC) to get the # of unix seconds, but when you call toString, it's automatically grabbing whatever TimeZone.getDefault() is set to (local system settings) and displaying that GMT unix seconds to you in a nice, pretty string adjusted according to your local system's offset from GMT. It's actually storing GMT.

I guess that's another possible approach that may be less subtle for users. now() always returns GMT, and we add the ability in Dates.format(dt, formatstring; local=true), so that it's explicit you want to adjust the output string to show what your current system is showing in the corner of your screen. I'd be ok with that.

Really though, I should just finish up TimeZones.jl and if a user is ever wading into territory of needing to deal with timezones, they should automatically hook into that.

[aviks]

cc: @quinnj @StefanKarpinski

[quinnj]

It's not the datetime2unix and unix2datetime functions, it's the use of Dates.now(). Dates.now() returns the local time as a DateTime type that inherently doesn't associate a timezone with it. When you call jcall(jd, "toString",JString,()), there's inherently a timezone tied to that Java date, so when you convert it to an integer, it's correctly shifting to GMT. We don't do that since there is no timezone inherently associated with our DateTime type. Hopefully that makes sense? You can call now(Dates.UTC) to get a DateTime according to GMT instead of local time which will correspond to your java dates and time().

[aviks]

This may be a matter of perspective, but I'm not particularly convinced yet.

I think it comes down to the definition of that 10 digit number. In both Java and Unix, it is defined as number of seconds since 1-1-1970 00:00:00 GMT. Not that the epoch is defined as GMT, not local time. Defining the epoch at GMT ensures that each instance in time, at any place on earth, can be described by the same number (modulo relativistic effects, or leap seconds). If you define epoch based on local time, then that number does not define an instance in time, you need a number and a location.

Hence I think Dates.datetime2unix(dt) should return a number based on this definition above. Which means that there is an implicit assumption about the timezone of dt in the datetime2unix call. And currently the assumption is that dt is in GMT, which is different from the assumption in now()

I think the inconsistency is best illustrated by the following invocation. I would expect that Dates.now() and unix's now converted via unix2datetime should return the same result. no?

julia> Dates.now()
2015-07-11T00:25:26

julia> Dates.unix2datetime(time())
2015-07-10T23:25:27.583

Having written all that however, I am not sure if full consistency is possible without the full TZ db.

PS: FWIW, the java Date type does not store any TZ information. Similar to julia the now() equivalent, which is the default constructor of that type, returns the current local time.

[StefanKarpinski]

I think I'm with Avik on this one. I'm also not entirely convinced about "naive time" in general.