An unrestricted duty is not tied by the Lawgiver to one fixed period after which its original mode of fulfilment necessarily expires.
The duty is to perform the required act, rather than to perform it only during a particular hour. Therefore missing a personally chosen time does not automatically convert the act into legal make-up performance.
For example, the obligation to pay zakat is not expressed as a duty that expires at one arbitrarily specified hour. Whether delay is allowed or blameworthy must be separately assessed.
Crucial distinction: “Unrestricted by time” does not mean “optional,” and allowance of deferred performance does not authorize permanent neglect.