Representation of date interval. A date interval stores either a fixed amount of time (in years, months, days, hours etc) or a relative time string in the format that DateTime's constructor supports.
Number of years.
Number of months.
Number of days.
Number of hours.
Number of minutes.
Number of seconds.
Is 1 if the interval is inverted and 0 otherwise. See DateInterval::format().
Total number of days the interval spans. If this is unknown, days will be FALSE.