组中日期之间的差异

Question

我想计算日期之间的差异（它们是字符，例如＆＃39; 2014-01-01＆＃39;天数和R中的组。我在DB2中做了类似的事情

DAYS(DATE(my_date)) - DAYS((LAG(my_date) OVER (PARTITION BY my_group ORDER BY blabla ))

my_group  my_date      diff
1         '2014-01-01'  NA
1         '2014-01-02'  1
1         '2014-01-05'  3
2         '2014-01-01'  NA
2         '2014-01-02'  1
2         '2014-01-06'  4
3         '2014-01-01'  NA
3         '2014-01-02'  1
3         '2014-01-10'  8
3         '2014-01-11'  1