Looks like it can't determine how long "first Monday of January" is.

echo $fmj->format('%y years, %m months, %d days, %h hours, %i minutes, %s seconds');

What numbers should it be using for those fields?

Not sure what you mean by the numbers you are looking for.

I got closer with this code (added "next year" to the DatePeriod object, but the result is erratic (not sure what is being calculated):


$fmj = DateInterval::createFromDateString('first Monday of January next year');

$period = new DatePeriod(new DateTime('first Monday of January 2020'), $fmj, new DateTime('2022-01-31'));

foreach ($period as $dt) {
    var_dump($dt->format("l Y-m-d H:i:s"));
}

// Strange, incorrect output (not the first Monday of January 2021 or 2022):
string(26) "Monday 2020-01-06 00:00:00"
string(29) "Wednesday 2021-01-06 00:00:00"
string(27) "Tuesday 2022-01-04 00:00:00"

$timezone = new DateTimeZone( "UTC" );
$fmj = DateInterval::createFromDateString( 'first Monday of January 2020' );
//Monday from start 2019 only
$period = new DatePeriod( new DateTime( '2019-01-01 00:00:00.000000', $timezone ), $fmj, new DateTime( '2022-01-11 00:00:00.000000', $timezone ) );

$y = 0;
foreach ( $period as $dt ) {

    $display[$y++] = $dt->format( "l Y-m-d H:i:s" );
		//stop total_recurrence
		if( $y == 5 ) {
			break;
		}
}
Manually stop infinite loop :)
The relative date, first Monday of jenaury is extended for the current year but for the parametres start and end of function DatePeriod , only uses the start, there are two bugs in the php source code, the reference is not counted if the code does not return the expected result, the second bug is stopping on start.

observe the behavior of DateTime, this code expands the relative date for the current year while for DatePeriod it must not expand for the current year but must use the value for the start and end year, it means if I have a year explicitly expressed DatePeriod result must be for the year expressed otherwise if the year is not expressed it recovers the start and end of DatePeriod.
$var = new DateTime( 'first monday of January', new DateTimeZone( "UTC" );
$display[5] = $var->format( "l Y-m-d H:i:s" );
var_dump( $display );