Uses incoming parameters to return a pair of POSIXct times in the
proper order. Both start and end times will have lubridate::floor_date()
applied to get the nearest unit. This can be modified by specifying
ceilingStart = TRUE or ceilingEnd = TRUE in which case
lubridate::ceiling_date() will be applied.
The required timezone parameter must be one of those found in
OlsonNames.
Dates can be anything that is understood by
lubrdiate::parse_date_time() including either of the following
recommended formats:
When startdate or enddate are already POSIXct values,
they are converted to the timezone specified by timezone without
altering the physical instant in time the input represents. This is different
from the behavior of parse_date_time (which powers
this function), which will force POSIXct inputs into a new timezone,
altering the physical moment of time the input represents.