title: How to get the first day of the current week and month?
body: <p>I have the date of several events expressed in <strong>milliseconds</strong>[1], and I want to know which events are inside the current week and the current month, but I can't figure out how to obtain the first day (day/month/year) of the running week and convert it to milliseconds, the same for the first day of the month.</p>
  
  <pre><code>[1]Since January 1, 1970, 00:00:00 GMT
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