Function shift_date 

pub fn shift_date(d: &DateValue, months: i64, seconds: f64) -> DateValue

Shift a date by a whole number of calendar months (the month field moves and the day is clamped to the new month’s length) and then by seconds. A date-only value stays date-only when the seconds are a whole number of days; otherwise it becomes a datetime in its offset (UTC for a date-only receiver).