Something is wrong with historical timezone changes.

[Reggino]

Thanks for this, awesome work!

I did however run into an issue: Considers the following code:

var moment = require('moment-timezone');

var date1 = moment('1940-05-16T02:00:00+02:00');
console.log(date1.format());
console.log(date1.startOf('day').format());

var date2 = moment('1940-05-16T00:00:00.000Z');
date2.tz('Europe/Amsterdam');
console.log(date2.format());
console.log(date2.startOf('day').format());

Expected output:

1940-05-16T02:00:00+02:00
1940-05-16T00:00:00+02:00
1940-05-16T02:00:00+02:00
1940-05-16T00:00:00+02:00

Actual output (in latest version):

1940-05-16T02:00:00+02:00
1940-05-16T00:00:00+02:00
1940-05-16T02:00:00+02:00
1940-05-15T23:20:00+00:20

1940-05-16 is a special date: this day the timezone changed from +00:20 to +02:00, See Wikipeda. It looks like startOf doesn't honor the timezone change correctly after being adjusted to the Europe/Amsterdam timezone?

[Reggino]

To make things more complicated, see this page with more details on the change. DST started on that exact date. As a result, on 00:00:00 the clock turned forward 1:40 hour . Maybe the last result should be: 1940-05-16T01:40:00+02:00?

[Reggino]

The same issue applies to other history related timezone changes. On 1937-07-01 the timezone changed from +01:19 to +01:20 in Europe/Amsterdam.

var moment = require('moment-timezone');

var date = moment('1937-07-01T00:00:00.000Z');
date.tz('Europe/Amsterdam');
console.log(date.format());
date.startOf('day');
console.log(date2.format());

has output

1937-07-01T01:20:00+01:20
1937-06-30T23:59:00+01:19

Where i expected

1937-07-01T01:20:00+01:20
1937-07-01T00:00:00+01:20

[mattjohnsonpint]

Yes, it appears there is something going on here.

moment.tz('1940-05-15', 'Europe/Amsterdam').format()
// "1940-05-15T00:00:00+00:20"  (correct)
moment.tz('1940-05-15', 'Europe/Amsterdam').startOf('day').format()
// "1940-05-15T00:00:00+00:20"  (correct)
moment('1940-05-15T00:00:00Z').tz('Europe/Amsterdam').format()
// "1940-05-15T00:20:00+00:20"  (correct)
moment('1940-05-15T00:00:00Z').tz('Europe/Amsterdam').startOf('day').format()
// "1940-05-15T00:00:00+00:20"  (correct)

moment.tz('1940-05-16', 'Europe/Amsterdam').format()
// "1940-05-16T01:40:00+02:00"  (correct)
moment.tz('1940-05-16', 'Europe/Amsterdam').startOf('day').format()
// "1940-05-16T02:00:00+02:00"  (incorrect - should be "1940-05-16T01:40:00+02:00")
moment('1940-05-16T00:00:00Z').tz('Europe/Amsterdam').format()
// "1940-05-16T02:00:00+02:00"  (correct)
moment('1940-05-16T00:00:00Z').tz('Europe/Amsterdam').startOf('day').format()
// "1940-05-15T23:20:00+00:20"  (incorrect - should be "1940-05-16T01:40:00+02:00")

[mattjohnsonpint]

Related issues: #327, moment/moment#3132, moment/moment#2749

[gilmoreorless]

This was fixed in moment v2.24.0, as long as you're using a version of moment-timezone that includes the full 1940 dataset.