Can the Bitcoin halving be delayed?
So were these 'premature' halvings due to the positive rate of change of hash rate?


Bitcoin is designed to keep the block time at 10 minutes. The difficulty increases/decreases as the mining hash rate increases/decreases. At this 10 minute rate the the halving event comes every 4 years. As miners started using ASIC chips to mine blocks, the average time came down and that led to slightly lower halving time.


  could a hash rate collapse lead to a delayed halving event (i.e. adding the current 210,000 blocks takes longer than 4 years)?


If hash rate declines continuously so that even after difficulty gets adjusted to account for lower hash rates, miners continue to lower their hash powers, then it could take longer than 4 years.