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Every second, the watchtower will attempt to decrypt the cipher using the current ISO 8601 time looking like "YYYY-mm-ddTHH:MM:SS" as the key.
This key would be very weak, you could use 64-bit UNIX timestamp, and it would be as weak as well (but then, at least it will be resistant to timezone issues).Quote
nobody will be able to use it unless they also know the unlock time, even if they have hacked the watchtower on a later date after the timer has started
Not really. Your program will need to decrypt it for every second, so your decryption could not take more time than that. The simplest way of getting the current time, and trying to decrypt it, can cause it to never be decrypted, if you will be unlucky, and your process will have a lower priority for a few seconds, when it should be decrypted.Another thing is, any attacker could scan it faster than one decryption per second, it could do 1000 decryptions per second, and reach it sooner. Also, if there will be some default locking time, for example two weeks, and the attacker will know that some file on your server was created one week ago (by checking metadata), then it will use one week offset, and scan only a range of time, and then will get to the solution much faster than the official algorithm.
I am coincidentally also working on a Lightning wallet (as long as it is written in Python as development of the wallet core has already begun).