title: Check if a date is within an interval in R
body: <p>I have these three intervals defined:  </p>
  
  <pre><code>YEAR_1  &lt;- interval(ymd('2002-09-01'), ymd('2003-08-31'))
  YEAR_2  &lt;- interval(ymd('2003-09-01'), ymd('2004-08-31')) 
  YEAR_3  &lt;- interval(ymd('2004-09-01'), ymd('2005-08-31'))
  </code></pre>
  
  <p>(in real life, I have 50 of these)</p>
  
  <p>I have a dataframe (called <code>df</code>) with a column full of lubridate formatted dates.  </p>
  
  <p>I'd like to append a new column on <code>df</code> which has the appropriate value <code>YEAR_n</code>, depending on which interval the date falls within.  </p>
  
  <p>Something like:  </p>
  
  <pre><code>df$YR &lt;- ifelse(df$DATE %within% YEAR_1, 1, NA)
  </code></pre>
  
  <p>but I'm not sure how to proceed. I need to somehow use an <code>apply</code> I think?</p>
  
  <p>Here's my dataframe:</p>
  
  <pre><code>structure(c(1055289600, 1092182400, 1086220800, 1074556800, 1109289600, 
  1041897600, 1069200000, 1047427200, 1072656000, 1048636800, 1092873600, 
  1090195200, 1051574400, 1052179200, 1130371200, 1242777600, 1140652800, 
  1137974400, 1045526400, 1111104000, 1073952000, 1052870400, 1087948800, 
  1053993600, 1039564800, 1141603200, 1074038400, 1105315200, 1060560000, 
  1072051200, 1046217600, 1107129600, 1088553600, 1071619200, 1115596800, 
  1050364800, 1147046400, 1083628800, 1056412800, 1159747200, 1087257600, 
  1201478400, 1120521600, 1066176000, 1034553600, 1057622400, 1078876800, 
  1010880000, 1133913600, 1098230400, 1170806400, 1037318400, 1070409600, 
  1091577600, 1057708800, 1182556800, 1091059200, 1058227200, 1061337600, 
  1034121600, 1067644800, 1039478400, 1022198400, 1063065600, 1096329600, 
  1049760000, 1081728000, 1016150400, 1029801600, 1059350400, 1087257600, 
  1181692800, 1310947200, 1125446400, 1057104000, NA, 1085529600, 
  1037664000, 1091577600, 1080518400, 1110758400, 1092787200, 1094601600, 
  1169424000, 1232582400, 1058918400, 1021420800, 1133136000, 1030320000, 
  1060732800, 1035244800, 1090800000, 1129161600, 1055808000, 1060646400, 
  1028678400, 1075852800, 1144627200, 1111363200, 1070236800), class = c("POSIXct", 
  "POSIXt"), tzone = "UTC")
  </code></pre>