functions for converting offset days to and from dates

day of month, bug: specifying 29, 30, or 31 means the dates will track the specific day of the month where possible, otherwise the day will be the last day of the month; so 31 will track the month end; also note that it is possible to start with e.g. (2024, 02, 31) and this will yield 2024-02-29 29 2024-03-31 2024-04-30 etc.

the date at the customer's location - ensure any time-zone conversion is performed before using this - as all calculations are date-only with no time component, summer time or other such time artefacts

a duration of a number of days

the offset of a date from the start date, in days