title: Select range of dates above and below another date
body: <p>Consider these small dataframes <code>a</code> and <code>b</code>:</p>
  
  <pre><code>a &lt;-structure(list(date = structure(c(16071, 16072, 16073, 16074, 16075, 
                                        16076, 16077, 16078, 16079, 16080, 
                                        16081), class = "Date"), 
                     value = c(3L, 5L, 6L, 6L, 15L, 2L, 7L, 12L, 20L, 22L, 100L)), 
                .Names = c("date", "value"), row.names = c(NA, -11L), class = "data.frame")
  
  b &lt;- structure(list(date = structure(c(16071, 16072, 16073, 16074, 16075, 
                                         16076, 16077, 16078, 16079, 16080, 16081), 
                                       class = "Date"), 
                      value = c(200L, 5L, 202L, 101L, 204L, 205L, 7L, 206L, 1000L, 
                                456L, 555L)), 
                 .Names = c("date", "value"), row.names = c(NA, -11L), 
                 class = "data.frame")
  </code></pre>
  
  <p>I want to create a third dataframe with the columns that match plus the two days above and two days below. For example my two values that match are (after merging a and b dataframes):</p>
  
  <pre><code>        date value
  1 2014-01-02     5
  2 2014-01-07     7
  </code></pre>
  
  <p>LETS DO ONLY ONE RECORD FROM ABOVE AND ONE FROM BELOW THE MATCHING RECORDS INSTEAD:
  The final dataset should have 1 records from below and 1 from above the matching records from both a and b:</p>
  
  <p>Something like so:</p>
  
  <p>FROM dataframe "a"</p>
  
  <p><code>date value
   1 2014-01-01     3 #one record above the matching record from dataframe "a"
   2 2014-01-02     5 #matching record from "a" and "b"
   3 2014-01-03     6 #one record below the matching record from dataframe "a"
   6 2014-01-06     2 #one record above the matching record from dataframe "a"
   7 2014-01-07     7 #matching record from "a" and "b"
   8 2014-01-08    12 #one record below the matching record from dataframe "a"</code></p>
  
  <p>FROM dataframe "b"</p>
  
  <p><code>date value
   1 2014-01-01   200  #one record from above
   2 2014-01-02     5  #matching record from dataframe "a" and "b"
   3 2014-01-03   202   #one record from below
   6 2014-01-06   205   #one record from above
   7 2014-01-07     7  #matching record from dataframe "a" and "b"
   8 2014-01-08   206 # one record from below</code></p>
  
  <p>FINAL PRODUCT SHOULD BE A COMBINATION OF BOTH a and b and would look like so:</p>
  
  <p><code>date value       date value
   1 2014-01-01     3 2014-01-01   200
   2 2014-01-02     5 2014-01-02     5
   3 2014-01-03     6 2014-01-03   202
   6 2014-01-06     2 2014-01-06   205
   7 2014-01-07     7 2014-01-07     7
   8 2014-01-08    12 2014-01-08   206</code></p>