title: Time series breaks not even on the half hour for an imported dataset
body: <p>I have been working on some analysis and have been grouping data into half hourly average groups for convenience as the data resolution is very fine (every 2 mins for a couple months). </p>
  
  <p>Data are imported as this:</p>
  
  <pre><code>unq_id    dat_tim      sens_hgt  leaf_temp_c  
     1    5/18/17 10:22      2      29.82043 
     2    5/18/17 10:24      2      32.27954 
     3    5/18/17 10:26      2      32.48996 
     4    5/18/17 10:28      2      31.81604 
     5    5/18/17 10:30      2      31.56943
  </code></pre>
  
  <p>The issue is that when I add a half hourly break class, the breaks are on a half hourly increment based off the first date-time measurement. Code used:</p>
  
  <pre><code>leaf_temp_df &lt;- read.csv("leaf_master.csv",header = TRUE, sep = ",")
  leaf_temp_df$halfhour &lt;- cut(as.POSIXct(paste(leaf_temp_df$dat_tim),
                                          format = "%m/%d/%y %H:%M"), breaks = "30 min")  
  </code></pre>
  
  <p>output:</p>
  
  <pre><code>unq_id    dat_tim     sens_hgt  leaf_temp_c        halfhour
     1   5/18/17 10:22      2      29.82043     2017-05-18 10:22:00
     2   5/18/17 10:24      2      32.27954     2017-05-18 10:22:00
     3   5/18/17 10:26      2      32.48996     2017-05-18 10:22:00
     4   5/18/17 10:28      2      31.81604     2017-05-18 10:22:00
     5   5/18/17 10:30      2      31.56943     2017-05-18 10:22:00
  </code></pre>
  
  <p>The output follows that pattern until it reaches the next break at 10:52:00</p>
  
  <p>I'd like the halfhour vector to be even on the half hour (e.g. 10:30:00 and 11:00:00) to enable compatibility between different data types. To fix this I tried to skip the first four lines of data to make unq_id = 5 the first line read as it begins on 10:30. </p>
  
  <pre><code>leaf_temp_df &lt;- read.csv("leaf_master.csv", header = TRUE, sep = ",")[-c(1:4),]
  </code></pre>
  
  <p>This still presents the same issue of the half hour breaks starting at 10:22. I've even tried to edit the master datafile and deleting lines 1-4 from the file to enable the breaks to read the first date time at an even half hour (10:30) but the 10:22 issue still presents itself.</p>