Dayjs: customParseFormat strict mode bug

[sheffieldnick]

Describe the bug
Using the format string "YYYY-MM-DD HH:mm:ss ZZ", parsing certain strings returns Invalid Date when it should return a date:

var format = "YYYY-MM-DD HH:mm:ss ZZ";
var ok = dayjs("2018-03-01 00:01:00 +0000", format, true);
var bad = dayjs("2018-04-01 00:01:00 +0000", format, true);

where bad is:

{
  "$D": 1
  "$H": 1
  "$L": "en"
  "$M": 3
  "$W": 0
  "$d": Invalid Date
  "$m": 1
  "$ms": 0
  "$s": 0
  "$u": undefined
  "$y": 2018
}

The parsing is successful if strict mode is not used. The problem seems to be in the customParseFormat.js line:

      if (isStrict && date !== this.format(format)) {

where the date that the plugin has parsed is not matching the default output from dayjs.format(). In the above case:

"2018-04-01 00:01:00 +0000" !== "2018-04-01 01:01:00 +0100"

but the same bug will effect parsing any date where the timezone doesn't match whatever local timezone/dst dayjs uses. eg. "2018-03-01 00:01:00 +0600", "2018-04-01 00:01:00 -0200", etc.

Expected behavior
bad should be a valid date object.

Information

• Day.js Version: v1.8.28
• OS: Windows 7
• Browser: Firefox 77, Chrome 83
• Time zone: UTC+00:00 DST (British Summer Time)

[z0al]

We have the same problem. I created this codesandbox example to reproduce the issue

[waspar]

R.I.P 👌

[DDoerner]

I've looked into the code and so far I see only two options for how this bug could be fixed:

1. The timezone information that is extracted by makeParser() could be passed back to the main method in addition to the parsed Date object. It can then be used to format the final result with a custom timezone before the string comparison is made. This however would mean to either require the timezone plugin for the customParseFormat plugin to work or to recreate this functionality in customParseFormat, which might be overkill
2. Rework how strict parsing in customParseFormat works by giving each segment expression information on how to enforce a strict format

[iamkun]

Bug confirmed. Strict mode does not work good with parsing timezone (ZZ)

I'm looking for a way to solve it.

[AllainPL]

I am trying to migrate some code from moment to dayjs, and this strict check is a big problem for me :(, any idea how to solve it?