How can I find the **last** value, **prior to test.day**, for each (`loc.x`, `loc.y`) pair?

```
dt <- data.table( 
  loc.x = as.integer(c(1, 1, 3, 1, 3, 1)),
  loc.y = as.integer(c(1, 2, 1, 2, 1, 2)),
  time = as.IDate(c("2015-03-11", "2015-05-10", "2015-09-27",
                    "2015-11-25", "2014-09-13", "2015-08-19")), 
  value = letters[1:6]
)

setkey(dt, loc.x, loc.y, time)
test.day <- as.IDate("2015-10-01")

```

Required output:

```
   loc.x loc.y value
1:     1     1     a
2:     1     2     f
3:     3     1     c

```