as_sys_time() converts x to a sys-time.
You can convert to a sys-time from any calendar type, as long as it has
at least day precision. There also must not be any invalid dates. If invalid
dates exist, they must first be resolved with invalid_resolve().
Converting to a sys-time from a naive-time retains the printed time, but adds an assumption that the time should be interpreted in the UTC time zone.
Converting to a sys-time from a zoned-time retains the underlying duration, but the printed time is the equivalent UTC time to whatever the zoned-time's zone happened to be.
Converting to a sys-time from a duration just wraps the duration in a sys-time object, adding the assumption that the time should be interpreted in the UTC time zone. The duration must have at least day precision.
There are convenience methods for converting to a sys-time from R's native date and date-time types. Like converting from a zoned-time, these retain the underlying duration, but will change the printed time if the zone was not already UTC.
Examples
x <- as.Date("2019-01-01")
# Dates are assumed to be naive, so the printed time is the same whether
# we convert it to sys-time or naive-time
as_sys_time(x)
#> <sys_time<day>[1]>
#> [1] "2019-01-01"
as_naive_time(x)
#> <naive_time<day>[1]>
#> [1] "2019-01-01"
y <- as.POSIXct("2019-01-01 01:00:00", tz = "America/New_York")
# The sys time displays the equivalent time in UTC (5 hours ahead of
# America/New_York at this point in the year)
as_sys_time(y)
#> <sys_time<second>[1]>
#> [1] "2019-01-01T06:00:00"
ym <- year_month_day(2019, 02)
# A minimum of day precision is required
try(as_sys_time(ym))
#> Error : Can't convert to a time point from a calendar with 'month' precision. A minimum of 'day' precision is required.
ymd <- set_day(ym, 10)
as_sys_time(ymd)
#> <sys_time<day>[1]>
#> [1] "2019-02-10"