Casting is one way to change a duration'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.
Casting to a more precise precision is done through a multiplication by a conversion factor between the current precision and the new precision.
A precision. One of:
x cast to the new
When you want to change to a less precise precision, you often want
duration_floor() instead of
duration_cast(), as that rounds towards
negative infinity, which is generally the desired behavior when working with
time points (especially ones pre-1970, which are stored as negative
x <- duration_seconds(c(86401, -86401)) # Casting rounds towards 0 cast <- duration_cast(x, "day") cast#> <duration<day>> #>  1 -1# Flooring rounds towards negative infinity floor <- duration_floor(x, "day") floor#> <duration<day>> #>  1 -2# Flooring is generally more useful when working with 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. as_sys_time(x)#> <time_point<sys><second>> #>  "1970-01-02 00:00:01" "1969-12-30 23:59:59"as_sys_time(cast)#> <time_point<sys><day>> #>  "1970-01-02" "1969-12-31"as_sys_time(floor)#> <time_point<sys><day>> #>  "1970-01-02" "1969-12-30"# Casting to a more precise precision duration_cast(x, "millisecond")#> <duration<millisecond>> #>  86401000 -86401000