Q:

Remove all dates prior to specific date in all years

How to remove all dates prior to a specific date, say, 15th of March?  
dates <- seq(as.Date("1990/1/1"), as.Date("1999/1/1"), "days")

I would need to subset a data frame based on this vector. 

A:

I would use lubridate for readability:
library(lubridate)
dates[month(dates) > 3 | (month(dates) == 3 & day(dates) >= 15)]

Base version  of the same logic:
dates[as.integer(format(dates, "%m")) > 3 |
    (as.integer(format(dates, "%m")) == 3 & as.integer(format(dates, "%d")) >= 15)]

