ADDDAYS is available since version 1.2.0.
Adds days to a timestamp or a tselements.
ADDDAYS function consumes from the top of the stack a number (positive or negative) of days, a time parameter (timestamp or tselements) and optionally a timezone parameter.
It adds the days and when an optional timezone is specified the computation is performed using it instead of
Timezone names are the ones defined in Joda Time.
This function will take into account leap years and, when working with timestamps, possible daylight saving time change.