Can block times be negative values?

or is this some kind of chain explorer bug?

here's two examples but I see a lot of them:

https://explorer.zcha.in/blocks/0000000338f7f41748307b0810f4e9dfe8ea15d57b80a3a4366736e6f218099a
https://explorer.zcha.in/blocks/00000002b6718ea88a3aa3d692f53607c74f3f3e15765fc86f9b773fc042b59f

Block timestamps are set by miners (whose hardware clocks may be out of sync) - at the moment Zchain just takes the difference of the timestamps, although seeing the frequency at which this is happening I think I might start recording the timestamps at which my Zcash nodes receive the blocks instead.