Implement some more delta steps

src/eval/util.rs

@@ -0,0 +1,19 @@
+use chrono::{Datelike, NaiveDate};
+
+pub fn is_leap_year(year: i32) -> bool {
+    NaiveDate::from_ymd_opt(year, 2, 29).is_some()
+}
+
+pub fn add_months(year: i32, month: u32, delta: i32) -> (i32, u32) {
+    let month0 = (month as i32) - 1 + delta;
+    let year = year + month0.div_euclid(12);
+    let month = month0.rem_euclid(12) as u32 + 1;
+    (year, month)
+}
+
+pub fn month_length(year: i32, month: u32) -> u32 {
+    NaiveDate::from_ymd_opt(year, month + 1, 1)
+        .unwrap_or_else(|| NaiveDate::from_ymd(year + 1, 1, 1))
+        .pred()
+        .day()
+}