Timelock
A condition, enforced by the bitcoin network itself, that keeps coins from moving until a set date or block, or until they've sat unmoved for a set time.
Bitcoin has two kinds, and both are part of the network’s rules, so no company, and not even the owner, can cut the wait short.
- Absolute: the coins can’t move before a given date or block. Wallet scripts have been able to require this since December 2015, under a standard called BIP65.
- Relative: the coins can’t move until they’ve sat unmoved for a given time, up to 65,535 blocks, about 15 months at the usual pace of one block every ten minutes. This came in July 2016, with BIP68 and BIP112.
A timelock can guard one path in a wallet rather than the whole wallet. A script can let two of three keys sign at any time, and one key alone only after a wait. That makes room for a backup key or an heir without handing anyone the money early, and if the owner keeps moving the coins, the clock starts over: a dead man’s switch.
The standards give their own examples. BIP112 describes three people, any two of whom can sign at once, and one of them alone after 30 days. BIP65 imagines two business partners whose lawyer holds a third key, and notes that the lawyer may prefer not to be able to use it at once, so that no one has a reason to try to force it out of them.
That’s also its limit against a robbery: a timelock slows only the path it’s written into. If another path, or your own key, can still move everything at once, a robber can make you use it. The guides on inheritance and on wrench attacks go further.