A minor issue is the comment:
 * returns    : Number of days strictly between from and to +1

in the first switch case if *from==*to 0 is returned so the comment can't be right. Note that the usage of this function depends on 0 being returned for *to==*from
------------------------------------------
The following problem is serious:
Breakpoint 1, yearfrac (from=0xbfffdd80, to=0xbfffdd78, 
    basis=GO_BASIS_MSRB_30_360) at gnm-datetime.c:158
158		if (!g_date_valid (from) || !g_date_valid (to))
(gdb) n
161		days = go_date_days_between_basis (from, to, basis);
(gdb) 
163		if (days < 0) {
(gdb) p days
$7 = -2
(gdb) p *from
$8 = {julian_days = 733831, julian = 1, dmy = 1, day = 28, month = 2, 
  year = 2010}
(gdb) p *to
$9 = {julian_days = 733831, julian = 1, dmy = 1, day = 28, month = 2, 
  year = 2010}
(gdb) c
Continuing.

Note that *from==*to so the return value ought to be 0 but is -2!

This bug underlies the first issue in bug 630784.