Can the Bitcoin halving be delayed? Currently the next halving date is May 2020 (provisional) according to enter link description here.

I understand that every 210k blocks, the block reward is halved and this works out, roughly, to once every 4 years. 

I guess I have two questions:
1) The 2012 halving took place approx. 3.8 years after genesis, and the 2016 halving took place a further 3.6 years after that. During this period hash rate increased almost continuously and at exaggerated rates due to continuous developments in mining technology from general to specific solutions (CPU -> GPU / FPGA -> ASIC). So were these 'premature' halvings due to the positive rate of change of hash rate?

2) A halving on 23 May 2020 is roughly 3.9 years from the last halving. It seems this is linked to 2018's drop in hash rate, could a hash rate collapse lead to a delayed halving event (i.e. adding the current 210,000 blocks takes longer than 4 years)?  

Thank you.
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.