Implement DATE statement

src/eval/error.rs

@@ -18,6 +18,14 @@ pub enum Error {
         start: NaiveDate,
         prev: NaiveDate,
     },
+    /// A `DATE`'s repeat delta did not move the date forwards in time. Instead,
+    /// it either remained at the current date (`to == from`) or moved backwards
+    /// in time (`to < from`).
+    RepeatDidNotMoveForwards {
+        span: Span,
+        from: NaiveDate,
+        to: NaiveDate,
+    },
     /// A `MOVE a TO b` statement was executed, but there was no entry at the
     /// date `a`.
     MoveWithoutSource { span: Span },