title: I need a method/function to efficiently convert weekly data to monthly data in R with fractional weeks
body: <p>I have time series data of several products in one data frame.
  Sample:
  columns: Name size volume date</p>
  <p>The volumes are measured every week
  I wish to convert this to a monthly estimate in a mathematical way, i.e not simply taking whatever week is in that month rather, for example - Lets start at Jan 1st, i take the first four weeks plus 3/7th of the fourth week to make 31 days and for the second month I take 4/7th of the split week plus whatever i need and so on.
  This is very tedious to do manually or even in a for loop.</p>
  <p>Is there a smarter way of doing this?
  Please help</p>
  <p>I had an idea but unsure of implementation, to spline the data and simply sum everything at the predicted end of the month. How can I do this?</p>