date_arg: Date, POSIX(c/l)t date/time, or parseable string in YYYY(/-)MM(/-)DD format, optionally with a space and HH:MM:SS in 24 hour time. Intended to be converted to ISO8601 string for use in Qualtrics API call
time_zone: Optional arg for time zone specification, defaults to system local timezone.
endofday: Boolean. If TRUE, if the entry has no time information (only date), then result will have 23:59:59 (end of day) versus the implicit 00:00:00 (start). Intended for, e.g., 'end_date' argument in fetch_survey(). This allows the intuitive expectation where entering end_date = 2022/05/05 includes rather than excludes cases on 05/05 itself.