.
Because March has more days than February the previous month offset was
getting normalized back to the current month--th 29 February this
year is really 1 March, so normalizing before setting the day caused
begin/end previous month to return the begin/end of the current month.
That probably happened on the 31st of May, July, October, and December
as well, I just hadn't managed to test on those days. Switching the
normalization to after calculating the day of the month broke the
previous quarter calculation because now the month was out of range, so
normalize month & year first.
After getting a better understanding of how it interacts with dialog-options and
finding some additional scheme API needing implementation (exposed only in
hello-world.scm, but possibly used in user-written reports).
Creates a new file-pair, gnc-options-date.[ch]pp to separate the creation of
the static structs with the type info and strings for the various
RelativeDatePeriod values from the already too large gnc-option-impl.
Although the result is a rather C-ish design the alternative with classes
or template specializations would be more complicated and offer no
benefit.
Implements the permissible_values functions of GncOptionMultichoiceValue
for GncDateValue and provides for absolute, relative, and both
UI values for the three widget configs afforded by dialog-options.
