HI,
Please check this link
(http://rss.acs.unt.edu/Rdoc/library/base/html/as.POSIXlt.html).
"Dates without times are treated as being at midnight UTC. "
May be you can try this (if it doesn't create additional problems):
dates.mine2<-dates.mine+1
dates.mine2
#[1] "2009-05-21 23:30:01 GMT" "2009-05-21 23:45:01 GMT"
#[3] "2009-05-22 00:00:01 GMT" "2009-05-22 00:15:01 GMT"
dates.mine2[3]
#[1] "2009-05-22 00:00:01 GMT"
str(dates.mine2)
#POSIXct[1:4], format: "2009-05-21 23:30:01" "2009-05-21 23:45:01" ...
A.K.