Convert TODAY and NOW to the workbook date system in 1904 workbooks (#578)

crates/engine/src/cf.rs

@@ -184,7 +184,7 @@ impl CfCache {
                 },
                 CfRule::TimePeriod { period, style } => match v.as_f64() {
                     Some(n) => {
-                        let today = gridcraft_calc::now_serial().floor();
+                        let today = gridcraft_core::date::serial_in_system(wb.date_system, gridcraft_calc::now_serial()).floor();
                         let d = n.floor();
                         let wd = gridcraft_core::date::datetime_from_serial(wb.date_system, today).map(|x| x.weekday as f64).unwrap_or(0.0);
                         let week_start = today - wd;

crates/engine/src/cmd/print.rs

@@ -391,7 +391,7 @@ pub struct HfContext {
 
 /// Today's date and the current time as headers and footers show them (`&D`, `&T`).
 pub fn hf_date_time(wb: &Workbook) -> (String, String) {
-    let now = gridcraft_calc::now_serial();
+    let now = gridcraft_core::date::serial_in_system(wb.date_system, gridcraft_calc::now_serial());
     let date = crate::display::format(&Value::Number(now.floor()), "m/d/yyyy", wb).text;
     let time = crate::display::format(&Value::Number(now), "h:mm AM/PM", wb).text;
     (date, time)
@@ -1765,6 +1765,14 @@ mod tests {
         }
     }
 
+    #[test]
+    fn header_footer_date_follows_the_workbook_date_system() {
+        let mut wb = Workbook::new();
+        let date_1900 = hf_date_time(&wb).0;
+        wb.date_system = gridcraft_core::DateSystem::D1904;
+        assert_eq!(hf_date_time(&wb).0, date_1900);
+    }
+
     #[test]
     fn header_footer_section_edits_are_undoable_and_views_are_per_sheet() {
         let mut s = s();