There ist the following build error for the both Debian repos:
[ 85s] There was 1 error:
[ 85s]
[ 85s] 1) Sabre\VObject\TimezoneUtilTest::testTimeZoneBCIdentifiers with data set #52 ('Canada/East-Saskatchewan')
[ 85s] Exception: DateTimeZone::__construct(): Unknown or bad timezone (Canada/East-Saskatchewan)
[ 85s]
[ 85s] /usr/src/packages/BUILD/tests/VObject/TimeZoneUtilTest.php:181

In a debian buglog I found the following:

"The root cause is that the timezome Canada/East-Saskatchewan is removed
in tzdata 2017c and should therefore be removed from php-bc.php"

But after I've reviewed "vendor/sabre/vobject/lib/timezonedata/php-bc.php" (I not firm to the buildsystem here, so I've searched somwhere else)
I've seen that in the newer php-bc.php there are now two separate timezones:
Canada/Eastern
and
Canada/Saskatchewan
So build should run again, if php-bc.php is modified in the sources.