ca19876 (for HHVM-130) removed the default (string) cast from the UTCDateTime constructor. This issue does not present itself with the PHP extension, as the constructor argument is currently parsed as a string (as of PHPC-726).
is related to
PHPC-804Timestamp and UTCDateTime constructors should accept integers in strict types mode
Closed
PHPC-726Allow cross-platform serialization of Timestamp and UTCDateTime