我有每日数据，我想从中提取每个月的最后一天。
原始数据：
Date Value
20 2008-01-28 82.55261
21 2008-01-29 83.43333
22 2008-01-30 83.07948
23 2008-01-31 84.22759
24 2008-02-01 85.77670
25 2008-02-04 84.87240
26 2008-02-05 82.58407
27 2008-02-06 81.77103
28 2008-02-07 80.78428
29 2008-02-08 81.51842
30 2008-02-11 82.39453
31 2008-02-12 84.09175
32 2008-02-13 85.58366
33 2008-02-14 83.77604
期望的输出：
## Date Value
## 1 2008-01-31 84.22759
## 2 2008-02-14 83.77604
G. Grothendieck 从这个问题的最早版本中发布的屏幕截图中整理了数据：
structure(list(Date = structure(c(13906, 13907, 13908, 13909,
13910, 13913, 13914, 13915, 13916, 13917, 13920, 13921, 13922,
13923), class …