at
Returns an instant that corresponds to the start of this date in the specified timeZone.
Note that it's not equivalent to atTime(0, 0).toInstant(timeZone) because a day does not always start at the fixed time 0:00:00. For example, if due do daylight saving time, clocks were shifted from 23:30 of one day directly to 00:30 of the next day, skipping the midnight, then atStartOfDayIn would return the Instant corresponding to 00:30, whereas atTime(0, 0).toInstant(timeZone) would return the Instant corresponding to 01:00.