Unlock Time

To spend an output the output's 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 500,000,000 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

Before hard-fork 13, this was done by just getting the computer's time, from hf 13 onwards, 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 block's 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.