Allow subsecond intervals with date_breaks()

[dpseidel]

Closes #85

This allows support for subsecond width arguments to work with date_breaks()

I also caught a bug whereby the to_time() function in floor_time() was mishandling the timezone of "min" and "sec" intervals, so I have adjusted that to match the tz handling for all other units of time.

[hadley]

Why didn't this work before? Because seq.POSIXct() doesn't support partial seconds? (even though it supports other partial units?)

[hadley]

Can you please indent so each argument to structure() goes on its own line?

[hadley]

Are you sure you need the %||% "" here?

[dpseidel]

I did this to match what is being called for other units in L32 but no I did not fully think through whether it's necessary in either place.

[hadley]

Ah ok, let's keep it consistent then

[dpseidel]

seq.POSIXct() doesn't support partial seconds that are represented by character strings. Only numerics.

options("digits.secs" = 2)
times <- as.POSIXct(c("2000-01-01 08:29:58", "2000-01-01 08:30:10"), tz = "UTC")
seq.POSIXt(times[1], times[2], by = "1 sec")
#>  [1] "2000-01-01 08:29:58 UTC" "2000-01-01 08:29:59 UTC"
#>  [3] "2000-01-01 08:30:00 UTC" "2000-01-01 08:30:01 UTC"
#>  [5] "2000-01-01 08:30:02 UTC" "2000-01-01 08:30:03 UTC"
#>  [7] "2000-01-01 08:30:04 UTC" "2000-01-01 08:30:05 UTC"
#>  [9] "2000-01-01 08:30:06 UTC" "2000-01-01 08:30:07 UTC"
#> [11] "2000-01-01 08:30:08 UTC" "2000-01-01 08:30:09 UTC"
#> [13] "2000-01-01 08:30:10 UTC"

seq.POSIXt(times[1], times[2], by = ".5 sec")
#> Error in seq.int(0, to0 - from, by): invalid '(to - from)/by'

seq.POSIXt(times[1], times[2], by = .5)
#>  [1] "2000-01-01 08:29:58.0 UTC" "2000-01-01 08:29:58.5 UTC"
#>  [3] "2000-01-01 08:29:59.0 UTC" "2000-01-01 08:29:59.5 UTC"
#>  [5] "2000-01-01 08:30:00.0 UTC" "2000-01-01 08:30:00.5 UTC"
#>  [7] "2000-01-01 08:30:01.0 UTC" "2000-01-01 08:30:01.5 UTC"
#>  [9] "2000-01-01 08:30:02.0 UTC" "2000-01-01 08:30:02.5 UTC"
#> [11] "2000-01-01 08:30:03.0 UTC" "2000-01-01 08:30:03.5 UTC"
#> [13] "2000-01-01 08:30:04.0 UTC" "2000-01-01 08:30:04.5 UTC"
#> [15] "2000-01-01 08:30:05.0 UTC" "2000-01-01 08:30:05.5 UTC"
#> [17] "2000-01-01 08:30:06.0 UTC" "2000-01-01 08:30:06.5 UTC"
#> [19] "2000-01-01 08:30:07.0 UTC" "2000-01-01 08:30:07.5 UTC"
#> [21] "2000-01-01 08:30:08.0 UTC" "2000-01-01 08:30:08.5 UTC"
#> [23] "2000-01-01 08:30:09.0 UTC" "2000-01-01 08:30:09.5 UTC"
#> [25] "2000-01-01 08:30:10.0 UTC"

[hadley]

Can you please add a comment explaining that seq() doesn't support partial seconds? Otherwise, is good to merge.