create date vector r
seq(as.Date("2010-1-1"), as.Date("2015-1-1"), by = "years")
## [1] "2010-01-01" "2011-01-01" "2012-01-01" "2013-01-01" "2014-01-01"
## [6] "2015-01-01"
seq(as.Date("2015/1/1"), as.Date("2015/12/30"), by = "quarter")
## [1] "2015-01-01" "2015-04-01" "2015-07-01" "2015-10-01"
seq(as.Date('2015-09-15'), as.Date('2015-09-30'), by = "2 days")
## [1] "2015-09-15" "2015-09-17" "2015-09-19" "2015-09-21" "2015-09-23"
## [6] "2015-09-25" "2015-09-27" "2015-09-29"