Can't get proper zoneName when no rule applies to the zone

[moment]

Can't get proper zoneName when no rule applies to the zone

[mattjohnsonpint]

This isn't a bug in moment-timezone. It's a hole in the tz database.

# Rule  NAME    FROM    TO      TYPE    IN      ON      AT      SAVE    LETTER/S
Rule    Japan   1948    only    -       May     Sun>=1  2:00    1:00    D
Rule    Japan   1948    1951    -       Sep     Sat>=8  2:00    0       S
Rule    Japan   1949    only    -       Apr     Sun>=1  2:00    1:00    D
Rule    Japan   1950    1951    -       May     Sun>=1  2:00    1:00    D

# Zone  NAME            GMTOFF  RULES   FORMAT  [UNTIL]
Zone    Asia/Tokyo      9:18:59 -       LMT     1887 Dec 31 15:00u
                        9:00    -       JST     1896
                        9:00    -       CJT     1938
                        9:00    Japan   J%sT

Asia/Tokyo is defined as anything after 1938 follows the Japan rule, but that rule is only defined from the first Sunday following May 1st 1948 (which happens to be May 2nd) forward.

[mattjohnsonpint]

Actually, I take back my last comment. I was misinterpreting the data. there is indeed a bug here.

Asia/Tokyo should follow the Japan rule should be followed for anything after 1938. From there forward it should be fixed to UTC+09:00 - except for those ranges defined in the rule, where it should follow the DST logic for those particular ranges.

Since 1948-05-01T16:59:59+00:00 is not in any range in the Japan rule, it doesn't follow DST, but it is still UTC+09:00 and should be considered "standard time".

Question - how can we assume that "S" is the correct letter to replace here?

[mattjohnsonpint]

Ok, I found a lot of good information here: How to read the tz database

In particular:

One wrinkle, not fully explained in zic.8.txt, is what happens when switching to a named rule. To what values should the SAVE and LETTER data be initialized?

• If at least one transition has happened, use the SAVE and LETTER data from the most recent.
• If switching to a named rule before any transition has happened, assume standard time (SAVE zero), and use the LETTER data from the earliest transition with a SAVE of zero.

We don't appear to be doing that. The replace is done in Zone.prototype.format:

format : function (rule) {
    return this.letters.replace("%s", rule.letters);
}

The problem is that in this case, the rule is empty. The format function needs to be expanded to consider the logic above.

[timrwood]

This issue kind of disappears when switching to the new data format.

[mattjohnsonpint]

Very nice. Thanks Tim!