GetString() returns inconsistent timezones

[chris-rs]

Very handy lib when working with recurring events!

When using $rule->getString(Rule::TZ_FIXED) and DTSTART is included in the string. The string returns different timezones for UNTIL and DTSTART:
For example:
"FREQ=WEEKLY;UNTIL=20180418T220000Z;DTSTART;TZID=Europe/Amsterdam:20180405T090000;BYDAY=MO".

DTSTART is in UTC+2 and UNTIL is UTC. This seems pretty inconsistent.
When looking at Rule.php UNTIL formats to UTC and DTSTART to included timezone info.

recurr/src/Recurr/Rule.php

Lines 411 to 449 in bff55c3

	// UNTIL or COUNT
	$until = $this->getUntil();
	$count = $this->getCount();
	if (!empty($until)) {
	if ($timezoneType === self::TZ_FIXED) {
	$u = clone $until;
	$u = $u->setTimezone(new \DateTimeZone('UTC'));
	$parts[] = 'UNTIL='.$u->format($format.'\Z');
	} else {
	$parts[] = 'UNTIL='.$until->format($format);
	}
	} elseif (!empty($count)) {
	$parts[] = 'COUNT='.$count;
	}
	// DTSTART
	if ($this->isStartDateFromDtstart) {
	if ($timezoneType === self::TZ_FIXED) {
	$d = $this->getStartDate();
	$tzid = $d->getTimezone()->getName();
	$date = $d->format($format);
	$parts[] = "DTSTART;TZID=$tzid:$date";
	} else {
	$parts[] = 'DTSTART='.$this->getStartDate()->format($format);
	}
	}
	// DTEND
	if ($this->endDate instanceof \DateTime) {
	if ($timezoneType === self::TZ_FIXED) {
	$d = $this->getEndDate();
	$tzid = $d->getTimezone()->getName();
	$date = $d->format($format);
	$parts[] = "DTEND;TZID=$tzid:$date";
	} else {
	$parts[] = 'DTEND='.$this->getEndDate()->format($format);
	}
	}

Why not using same timezone, just UTC, for all date formats in the string?