title: R: count 15 minutes interval in time
body: <p>I would like to count the amount of sessions started at  each 15 minutes intervals for businessdays within a large dataset. </p>
  
  <p>My data looks like: </p>
  
  <pre><code>df &lt;- 
  
  Start_datetime       End_datetime       Duration    Volume
  2016-04-01 06:20:55 2016-04-01 14:41:22  08:20:27   8.360
  2016-04-01 08:22:27 2016-04-01 08:22:40  00:00:13   0.000
  2016-04-01 08:38:53 2016-04-01 09:31:58  00:53:05   12.570
  2016-04-01 09:33:57 2016-04-01 12:37:43  03:03:46   7.320
  2016-04-01 10:05:03 2016-04-01 16:41:16  06:36:13   9.520
  2016-04-01 12:07:57 2016-04-02 22:22:32  34:14:35   7.230
  2016-04-01 16:56:55 2016-04-02 10:40:17  17:43:22   5.300
  2016-04-01 17:29:18 2016-04-01 19:50:29  02:21:11   7.020
  2016-04-01 17:42:39 2016-04-01 19:45:38  02:02:59   2.430
  2016-04-01 17:47:57 2016-04-01 20:26:35  02:38:38   8.090
  2016-04-01 22:00:15 2016-04-04 08:22:21  58:22:06   4.710
  2016-04-02 01:12:38 2016-04-02 09:49:00  08:36:22   3.150
  2016-04-02 01:32:00 2016-04-02 12:49:47  11:17:47   5.760
  2016-04-02 07:28:48 2016-04-04 06:58:56  47:30:08   0.000
  2016-04-02 07:55:18 2016-04-05 07:55:15  71:59:57   0.240
  </code></pre>
  
  <p>I would like to count all the starting sessions per 15minutes starting, where:</p>
  
  <pre><code>For business days
    Time                PTU    Count
    00:00:00 - 00:15:00  1       10     #(where count is the amount of sessions started between 00:00:00 and 00:15:00)
    00:15:00 - 00:30:00  2       6
    00:30:00 - 00:45:00  3       5
    00:45:00 - 01:00:00  3       3
  </code></pre>
  
  <p>And so on and the same data for the weekend. </p>
  
  <p>I have tried the cut function: </p>
  
  <pre><code>df$PTU &lt;- table (cut(df$Start_datetime, breaks="15 minutes"))
  data.frame(PTU)
  </code></pre>
  
  <p>EDIT: When I run this i receive the following error: </p>
  
  <pre><code>Error in cut.default(df$Start_datetime, breaks = "15 minutes") :'x' must be numeric
  </code></pre>
  
  <p>And some functions with lubridate, but I can't seem to make it work. My final goal is to create a table like the following, but then with 15 minutes interval.<br>
  <a href="https://i.stack.imgur.com/FggI9.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/FggI9.png" alt="enter image description here"></a></p>