Was caused by r71751, when the check for if the second argument to wfTimestamp is 0 changed from == to === sign. The constructor to Block has $this->mTimestamp = wfTimestamp( TS_MW, $timestamp ); in it, with $timestamp defaulting to '' if unspecified.

I'm not sure if wfTimestamp should be changed to use == so that '' == 0 or if Block::__construct should be changed to use 0 instead of '', or if both should be done.

fixed in r79013 (I changed the block constructor to use 0 instead of '')