Explain the results of ceiling Dates?

[bigdataman2015]

julia> using Dates

julia> ceil(DateTime(2013, 2, 13, 0, 31, 24), Dates.Minute(15))
2013-02-13T00:45:00

julia> ceil(DateTime(2013, 2, 13, 0, 31, 24), Dates.Minute(16))
2013-02-13T00:32:00

julia> ceil(DateTime(2013, 2, 13, 0, 31, 24), Dates.Minute(17))
2013-02-13T00:33:00

julia> ceil(DateTime(2013, 2, 11, 0, 31, 24), Dates.Minute(17))
2013-02-11T00:40:00

in R
library(lubridate)
ceiling_date(ymd_hms(“2013-02-13 00:31:24”), “15 mins”)
[1] “2013-02-13 00:45:00 UTC”
ceiling_date(ymd_hms(“2013-02-13 00:31:24”), “16 mins”)
[1] “2013-02-13 00:32:00 UTC”
ceiling_date(ymd_hms(“2013-02-13 00:31:24”), “17 mins”)
[1] “2013-02-13 00:34:00 UTC”
ceiling_date(ymd_hms(“2013-02-11 00:31:24”), “17 mins”)
[1] “2013-02-11 00:34:00 UTC”

explain the results of ceiling Dates ? thanks!