get_date_from_gmt returns time=zero when timezone is not set

[goodevilgenius]

when calling get_date_from_gmt(null) when the timezone is not set (i.e., get_option('timezone_string') returns null), the function returns Thu, 01 Jan 1970 00:00:00 +0000.

It should return the current time in the server's default timezone.

On line 2283 of formatting.php, we have this line:

return date( $format, 0 );

This should be replaced with:

return date( $format );

[SergeyBiryukov]

Introduced in [22435] for get_gmt_from_date(), copied to get_date_from_gmt() in [23618].

[owencutajar]

I get the same issue when timezone is set to UTC+0 .. function returns Thu, 01 Jan 1970 00:00:00 +0000 instead of current time.

[Rarst]

I am hesitant about issue as reported. The function is clearly documented to accept non-nullable string in Y-m-d H:i:s format. Passing it null does not seem to be either explicitly or implicitly meaningful.

Returning "zero" date is a case for invalid input, which is confusing as for me, but that's how it is written as of right now. I do not see how returning current time for invalid input is especially more meaningful or desired. It’s an error case. If anything it should probably fail altogether on it, but backwards compatibility.

What are the good reasons here to change behavior in such way?

[Rarst]

Needs wp_timezone() merged.

[Rarst]

I have added unit tests and rewrote all three of date converting functions. They were definitely pretty broken before.

I've kept the "zero" date return case, because, well, it's in there and I don't know what out there might be possibly relying on it by now.

[SergeyBiryukov]

In 45887:

Date/Time: Rewrite and simplify get_gmt_from_date(), get_date_from_gmt(), and iso8601_to_datetime() using wp_timezone().

Improve unit test coverage.

Props Rarst, goodevilgenius.
Fixes #31809.