Authorizing access to one or more restricted functions provided by a processing device is described. A method for authorizing such access comprises: reading at least one time from a clock associated with the processing device; storing the read time as a reference time; after the read time is stored, reading another time (a current time) from the clock. The method also includes comparing the reference time with the current time; reading a trend status associated with the reference time; and authorizing access to the restricted functions provided by the processing device if an acceptance criterion is satisfied. The acceptance criterion may require (i) the read current time to be chronologically earlier than the reference time and the trend status to be trusted backward mode, or (ii) the read current time to be chronologically later than the reference time.