The lag order, by, is defined the opposite way as in R base. Thus, -1 is a
lead and +1 a lag.
If by is integer, the time stamp is shifted by the number of periods. This
requires the series to be regular.
If by is character, the time stamp is shifted by a specific amount of time.
This can be one of one of "sec", "min", "hour", "day", "week",
"month", "quarter" or "year", optionally preceded by a (positive or
negative) integer and a space, or followed by plural "s". This is passed to
base::seq.Date(). This does not require the series to be regular.