Unlock Time
To spend an output the outputs unlock time must have passed.
Interpreting An Unlock Time
The unlock time is just a 64 bit unsigned number. It is interpreted as a block height if less than 500000000 otherwise it's a Unix timestamp1.
Checking The Output Is Unlocked
Block Height
First you get the top blocks height and add one, we do this because we are checking if the transaction is allowed in the next block not the last.
We now check if this height is greater than or equal to the unlock time if it is then accept the block2.
Timestamp
Getting The Current Time
Pre hard-fork 13 this was done by just getting the computers time, from hf 13 we use an average over the last blocks3.
Monero uses the last 60 blocks to get an average, if the chain height is less than
60 just use the current time4.
First you get the median timestamp of the last 60 blocks we then project this timestamp to match approximately when the block being validated will appear, to do this we do5:
\(adjustedMedian = median + \frac{(TimestampWindow + 1) * DifficultyTarget}{2} \)
where:
\(TimestampWindow = 60\)
\(DifficultyTarget = 120\)
You then get the top blocks timestamp and add the target seconds per block6.
The timestamp we use is then the minimum out of the adjusted median and adjusted most recent timestamp7.
Checking Timestamp Has Passed
Now with our timestamp we add the target seconds per block and check if this is more than or equal to the unlock time8.