Casting is one way to change a time point's precision.
Casting to a less precise precision will completely drop information that
is more precise than the precision that you are casting to. It does so
in a way that makes it round towards zero. When converting time points
to a less precise precision, you often want time_point_floor() instead
of time_point_cast(), as that handles pre-1970 dates (which are
stored as negative durations) in a more intuitive manner.
Casting to a more precise precision is done through a multiplication by a conversion factor between the current precision and the new precision.
Examples
# Hour precision time points
# One is pre-1970, one is post-1970
x <- duration_hours(c(25, -25))
x <- as_naive_time(x)
x
#> <naive_time<hour>[2]>
#> [1] "1970-01-02T01" "1969-12-30T23"
# Casting rounds the underlying duration towards 0
cast <- time_point_cast(x, "day")
cast
#> <naive_time<day>[2]>
#> [1] "1970-01-02" "1969-12-31"
# Flooring rounds the underlying duration towards negative infinity,
# which is often more intuitive for time points.
# Note that the cast ends up rounding the pre-1970 date up to the next
# day, while the post-1970 date is rounded down.
floor <- time_point_floor(x, "day")
floor
#> <naive_time<day>[2]>
#> [1] "1970-01-02" "1969-12-30"
# Casting to a more precise precision, hour->millisecond
time_point_cast(x, "millisecond")
#> <naive_time<millisecond>[2]>
#> [1] "1970-01-02T01:00:00.000" "1969-12-30T23:00:00.000"