Documentation: Additional information on difference-between

[SFRailsback]

This stuff needs to be covered in the documentation for difference-between. When used on logotimes of type "DAY":

• It is an error to ask for differences in years (time:difference-between logotime1 logotime2 "YEAR")
• Differences in units of "DAY" are simply the Julian date (day of the year) of logotime2 minus the Julian date of logotime1, using the year 2000. This means that leap day (February 29) is included. Therefore this code: time:difference-between (time:create "02-28") (time:create "03-01") "DAY" produces a result of 2 days. This method also means that difference-between can produce results between -365 and +365 days.

When used on logotimes of type "DATE", differences in units of "DAY" are the absolute number of days between the two dates, so they can be > 365 or <-365.

[charlesres]

@SFRailsback, thanks for submitting. I'll add those changes to the documentation, especially adding the rationale for not supporting year for "DAY". There are some interesting behaviors with addition that causes values to truncate or forget values after being converted that make years difficult to support.