Detailed Instructions: In this task, you're given passages that contain mentions of a time duration related quesry and we are supposed to write answer to a question that involves event “frequency", which refers to how often an event is likely to be repeated. For example, "taking showers" typically occurs ~5 times a week, "going to saturday market" usually happens every few weeks/months, etc. 
 Note that a lot of the questions could have more than one correct answers. We only need a single most-likely answer. Please try to keep your "answer" as simple as possible. Concise and simple "answer" is preferred over those complex and verbose ones.  
Q: Sentence: Some clock-shift problems could be avoided by adjusting clocks continuously or at least more gradually—for example, Willett at first suggested weekly 20-minute transitions—but this would add complexity and has never been implemented. 
Question: Willett's first suggestion was to change clocks by 20 minutes how often?
A:
weekly