index(oil_monthly) <- seq(mdy('01/01/2006'), mdy('02/01/2017'), by = 'months')
head(index(oil_monthly))
## [1] "2006-01-01" "2006-02-01" "2006-03-01" "2006-04-01" "2006-05-01"
## [6] "2006-06-01"