title: R - How to perform cross-year date operations?
body: <p>I am working with daily measurements of temperature. In total I have about 40 years of observations. How can I perform date operations covering a time interval that crosses years?</p>
  
  <p>For example, I want to sum the values from every october-to-february period. However, the sum should be taken only on the <strong>contiguous</strong> period of oct-nov-dec-jan-feb.</p>
  
  <p>"Isolated" months should not be taken into account, like for example jan and feb of the first year, and oct-nov-dec of the last year. The sum has to run over the contiguous period only (from oct-nov-dec-jan-fev). </p>
  
  <p>For example, this is what I am looking for:</p>
  
  <pre><code>        1st year                     2nd year                      3rd year
  J-F-M-A-M-J-J-A-S-**O-N-D   J-F**-M-A-M-J-J-A-S-**O-N-D   J-F**-M-A-M-J-J-A-S-O-N-D
  </code></pre>
  
  <p>But this is not OK:</p>
  
  <pre><code>        1st year                          2nd year                      3rd year
  **J-F**-M-A-M-J-J-A-S-**O-N-D   J-F**-M-A-M-J-J-A-S-**O-N-D   J-F**-M-A-M-J-J-A-S-**O-N-D**
  </code></pre>
  
  <p>This is a sample data frame to work on:</p>
  
  <pre><code>df &lt;- structure(list(date = structure(c(-3653, -3622, -3593, -3562, 
  -3532, -3501, -3471, -3440, -3409, -3379, -3348, -3318, -3287, 
  -3256, -3228, -3197, -3167, -3136, -3106, -3075, -3044, -3014, 
  -2983, -2953, -2922, -2891, -2863, -2832, -2802, -2771, -2741, 
  -2710, -2679, -2649, -2618, -2588, -2557, -2526, -2498, -2467, 
  -2437, -2406, -2376, -2345, -2314, -2284, -2253, -2223, -2192, 
  -2161, -2132, -2101, -2071, -2040, -2010, -1979, -1948, -1918, 
  -1887, -1857, -1826, -1795, -1767, -1736, -1706, -1675, -1645, 
  -1614, -1583, -1553, -1522, -1492, -1461, -1430, -1402, -1371, 
  -1341, -1310, -1280, -1249, -1218, -1188, -1157, -1127, -1096, 
  -1065, -1037, -1006, -976, -945, -915, -884, -853, -823, -792, 
  -762, -731, -700, -671, -640, -610, -579, -549, -518, -487, -457, 
  -426, -396, -365, -334, -306, -275, -245, -214, -184, -153, -122, 
  -92, -61, -31, 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 
  334, 365, 396, 424, 455, 485, 516, 546, 577, 608, 638, 669, 699, 
  730, 761, 790, 821, 851, 882, 912, 943, 974, 1004, 1035, 1065, 
  1096, 1127, 1155, 1186, 1216, 1247, 1277, 1308, 1339, 1369, 1400, 
  1430, 1461, 1492, 1520, 1551, 1581, 1612, 1642, 1673, 1704, 1734, 
  1765, 1795, 1826, 1857, 1885, 1916, 1946, 1977, 2007, 2038, 2069, 
  2099, 2130, 2160, 2191, 2222, 2251, 2282, 2312, 2343, 2373, 2404, 
  2435, 2465, 2496, 2526, 2557, 2588, 2616, 2647, 2677, 2708, 2738, 
  2769, 2800, 2830, 2861, 2891, 2922, 2953, 2981, 3012, 3042, 3073, 
  3103, 3134, 3165, 3195, 3226, 3256, 3287, 3318, 3346, 3377, 3407, 
  3438, 3468, 3499, 3530, 3560, 3591, 3621, 3652, 3683, 3712, 3743, 
  3773, 3804, 3834, 3865, 3896, 3926, 3957, 3987, 4018, 4049, 4077, 
  4108, 4138, 4169, 4199, 4230, 4261, 4291, 4322, 4352, 4383, 4414, 
  4442, 4473, 4503, 4534, 4564, 4595, 4626, 4656, 4687, 4717, 4748, 
  4779, 4807, 4838, 4868, 4899, 4929, 4960, 4991, 5021, 5052, 5082, 
  5113, 5144, 5173, 5204, 5234, 5265, 5295, 5326, 5357, 5387, 5418, 
  5448, 5479, 5510, 5538, 5569, 5599, 5630, 5660, 5691, 5722, 5752, 
  5783, 5813, 5844, 5875, 5903, 5934, 5964, 5995, 6025, 6056, 6087, 
  6117, 6148, 6178, 6209, 6240, 6268, 6299, 6329, 6360, 6390, 6421, 
  6452, 6482, 6513, 6543, 6574, 6605, 6634, 6665, 6695, 6726, 6756, 
  6787, 6818, 6848, 6879, 6909, 6940, 6971, 6999, 7030, 7060, 7091, 
  7121, 7152, 7183, 7213, 7244, 7274), class = "Date"), temp = c(22.9223529411765, 
  23.0705882352941, 23.1094117647059, 20.7835294117647, 17.4517647058824, 
  17.3176470588235, 18.0494117647059, 19.6188235294118, 21.3023529411765, 
  23.1105882352941, 22.2364705882353, 22.7482352941176, 23.5870588235294, 
  24.0023529411765, 23.0094117647059, 22.0176470588235, 19.4917647058824, 
  18.1011764705882, 18.3164705882353, 20.0623529411765, 22.8717647058824, 
  23.2576470588235, 23.68, 22.3694117647059, 22.9517647058824, 
  23.6976470588235, 23.3294117647059, 20.8564705882353, 18.16, 
  15.8988235294118, 15.7988235294118, 18.4176470588235, 20.8423529411765, 
  20.3247058823529, 22.3070588235294, 22.2035294117647, 24.2235294117647, 
  23.6976470588235, 24.4082352941176, 21.1752941176471, 18.1023529411765, 
  16.1211764705882, 18.3164705882353, 19.7635294117647, 23.1294117647059, 
  22.9964705882353, 23.6552941176471, 22.6964705882353, 23.6011764705882, 
  23.6517647058824, 23.7035294117647, 22.4352941176471, 18.5835294117647, 
  16.5976470588235, 15.7741176470588, 19.2541176470588, 20.8776470588235, 
  20.5729411764706, 21.1729411764706, 21.5870588235294, 22.4576470588235, 
  23.6058823529412, 21.84, 21.6694117647059, 19.2458823529412, 
  18.7517647058824, 17.7811764705882, 19.4764705882353, 21.9270588235294, 
  21.5470588235294, 22.88, 23.2458823529412, 24.2776470588235, 
  25.2470588235294, 23.4694117647059, 21.4435294117647, 19.3941176470588, 
  18.5447058823529, 17.6, 18.3764705882353, 19.8529411764706, 22.0823529411765, 
  22.7294117647059, 23.4011764705882, 23.3611764705882, 24.2505882352941, 
  23.2870588235294, 21.9482352941176, 20.5552941176471, 18.0788235294118, 
  18.5929411764706, 20.8752941176471, 21.9023529411765, 23.6105882352941, 
  22.4070588235294, 21.5635294117647, 23.3129411764706, 22.9741176470588, 
  23.3670588235294, 19.6105882352941, 16.9941176470588, 17.7670588235294, 
  17.4858823529412, 17.8517647058824, 20.26, 22.1576470588235, 
  23.8364705882353, 23.4447058823529, 24.8129411764706, 25.1764705882353, 
  24.2694117647059, 21.5035294117647, 20.0458823529412, 18.4694117647059, 
  18.4541176470588, 19.5388235294118, 22.02, 20.5364705882353, 
  22.9858823529412, 21.9752941176471, 23.7729411764706, 24.0576470588235, 
  24.0941176470588, 22.1552941176471, 21.2329411764706, 19.5611764705882, 
  17.8788235294118, 18.6823529411765, 20.1541176470588, 21.6258823529412, 
  21.5211764705882, 23.9811764705882, 24.8352941176471, 24.5882352941176, 
  24.1729411764706, 21.1035294117647, 19.0435294117647, 17.08, 
  17.4529411764706, 19.1458823529412, 20.4447058823529, 20.7129411764706, 
  21.5047058823529, 22.6952941176471, 23.4364705882353, 23.1, 24.1847058823529, 
  19.8105882352941, 19.9847058823529, 20.5188235294118, 17.7658823529412, 
  19.4435294117647, 20.7588235294118, 21.7835294117647, 22.7788235294118, 
  23.2388235294118, 24.9129411764706, 25.6, 23.5647058823529, 24.0058823529412, 
  19.7823529411765, 19.3152941176471, 18.7741176470588, 19.0305882352941, 
  20.5576470588235, 21.3611764705882, 21.4247058823529, 23.4811764705882, 
  23.6505882352941, 25.1870588235294, 23.3541176470588, 21.4823529411765, 
  18.7364705882353, 17.7235294117647, 18.3976470588235, 19.7235294117647, 
  21.0741176470588, 21.6094117647059, 22.9635294117647, 22.4011764705882, 
  23.4152941176471, 24.7741176470588, 24.3270588235294, 20.7976470588235, 
  18.8764705882353, 17.7788235294118, 16.4129411764706, 21.4117647058824, 
  22.3317647058824, 21.66, 22.3694117647059, 23.0917647058824, 
  24.4541176470588, 23.2847058823529, 23.3164705882353, 21.2529411764706, 
  19.1258823529412, 17.3882352941176, 17.3823529411765, 19.0529411764706, 
  19.6576470588235, 20.2976470588235, 21.9023529411765, 23.3094117647059, 
  24.0117647058824, 25.5611764705882, 24.9129411764706, 21.3964705882353, 
  19.9870588235294, 18.3929411764706, 20.9917647058824, 20.3058823529412, 
  21.4435294117647, 23.1941176470588, 22.8388235294118, 22.5176470588235, 
  24.6317647058824, 24.6541176470588, 24.2, 20.84, 18.4576470588235, 
  17.5011764705882, 19.16, 20.54, 20.1517647058824, 22.6776470588235, 
  22.7470588235294, 22.7882352941176, 22.0811764705882, 24.2152941176471, 
  22.9235294117647, 20.8411764705882, 19.6188235294118, 17.16, 
  16.0529411764706, 20.3223529411765, 19.9752941176471, 22.5152941176471, 
  22.2705882352941, 23.1541176470588, 23.1047058823529, 23.9517647058824, 
  24.8176470588235, 22.18, 20.5023529411765, 17.3505882352941, 
  19.1917647058824, 19.9894117647059, 19.0235294117647, 22.8235294117647, 
  22.7094117647059, 23.8741176470588, 24.0517647058824, 25.1764705882353, 
  23.9235294117647, 21.2929411764706, 20.6117647058824, 17.1305882352941, 
  16.3470588235294, 19.6470588235294, 21.3341176470588, 20.2176470588235, 
  23.7435294117647, 22.6741176470588, 22.9070588235294, 24.7152941176471, 
  23.2905882352941, 20.5776470588235, 18.9635294117647, 19.0658823529412, 
  18.8423529411765, 20.0729411764706, 21.3047058823529, 22.1588235294118, 
  24.0388235294118, 22.1917647058824, 24.0517647058824, 24.8729411764706, 
  23.0117647058824, 23, 21.3094117647059, 19.4105882352941, 20.3470588235294, 
  19.4482352941176, 20.0670588235294, 21.6364705882353, 23.4211764705882, 
  23.16, 25.4788235294118, 26.4741176470588, 24.0482352941176, 
  21.4176470588235, 21.7164705882353, 19.0905882352941, 19.6752941176471, 
  18.1611764705882, 20.0482352941176, 23.4917647058824, 23.4894117647059, 
  22.5482352941176, 23.1376470588235, 24.9811764705882, 24.1552941176471, 
  22.8423529411765, 19.7435294117647, 16.4, 17.3105882352941, 20.5235294117647, 
  21.0494117647059, 23.1352941176471, 23.9435294117647, 23.9058823529412, 
  24.9835294117647, 24.6952941176471, 24.0047058823529, 23.3164705882353, 
  21.5823529411765, 18.3447058823529, 18.1964705882353, 20.0035294117647, 
  20.7152941176471, 22.5705882352941, 24.6541176470588, 23.2329411764706, 
  25.0517647058824, 24.3329411764706, 23.5811764705882, 22.9988235294118, 
  19.4976470588235, 17.3188235294118, 19.5635294117647, 19.0211764705882, 
  19.7223529411765, 22.6858823529412, 23.9423529411765, 23.6905882352941, 
  25.7129411764706, 23.9505882352941, 24.4376470588235, 22.6070588235294, 
  19.8882352941176, 17.2058823529412, 16.4211764705882, 20.02, 
  21.9458823529412, 21.9341176470588, 22.74, 23.8, 23.9611764705882, 
  24.4564705882353, 24, 23.2129411764706, 19.4729411764706, 17.7105882352941, 
  16.9682352941176, 19.0341176470588, 20.2917647058824, 20.7776470588235, 
  22.9364705882353, 22.7894117647059)), .Names = c("date", "temp"
  ), row.names = c(NA, -360L), class = "data.frame")
  </code></pre>
  
  <p>Any input appreciated.</p>