[DennisP]

Here's a spec from 2015:

https://github.com/ethereum/wiki/blob/c02254611f218f43cbb075...

From the block validation section: "Is block.timestamp <= now + 900 and is block.timestamp >= parent.timestamp?"

(In Solidity, block.timestamp is in seconds, and I think that's a direct translation from the underlying opcode, so I assume it's 900 seconds here.)

[Drdrdrq]

I stand corrected - it seems blocks' time must be increasing and must not be more than 15 minutes in the future, according to clocks of validating miners. Thanks, didn't know that!