Add DateTime type and operations

This commit introduces the `DateTime` type to the language, enabling
users to work with dates and times. It includes:

- A new `DateTime` variant in the `Value` and `StaticType` enums.
- A `date` function for parsing date strings into `DateTime` values.
- Overloads for `+` and `-` operators to support `DateTime` arithmetic.
- Comparison operators (`>`, `<`) for `DateTime` values.
- Corresponding unit tests for `DateTime` operations.
- Updates to the `gemini.md` documentation to reflect the new
  functionality.
  Add DateTime type and operations

Introduce a new `DateTime` type to the language, allowing for date and
time manipulation. This includes:

*   Parsing dates and datetimes from strings.
*   Performing arithmetic operations (addition and subtraction) between
    `DateTime` and `Int` (representing milliseconds) and between two
    `DateTime` values (resulting in an `Int` duration).
*   Enabling comparison operations (`>`, `<`) between `DateTime` values.

src/ast/environment.rs

@@ -89,11 +89,31 @@ impl Environment {
 
     fn register_stdlib(&self) {
         use crate::ast::types::Signature;
+        use chrono::{NaiveDate, NaiveDateTime};
+
+        self.register_native("date", StaticType::Function(Box::new(Signature {
+            params: vec![StaticType::Text],
+            ret: StaticType::DateTime
+        })), |args| {
+            if let Value::Text(s) = &args[0] {
+                // Try parse YYYY-MM-DD
+                if let Ok(dt) = NaiveDate::parse_from_str(s, "%Y-%m-%d") {
+                    let ts = dt.and_hms_opt(0, 0, 0).unwrap().and_utc().timestamp_millis();
+                    return Value::DateTime(ts);
+                }
+                // Try parse YYYY-MM-DD HH:MM:SS
+                if let Ok(dt) = NaiveDateTime::parse_from_str(s, "%Y-%m-%d %H:%M:%S") {
+                    return Value::DateTime(dt.and_utc().timestamp_millis());
+                }
+            }
+            Value::Void
+        });
 
         let plus_ty = StaticType::FunctionOverloads(vec![
             Signature { params: vec![StaticType::Int, StaticType::Int], ret: StaticType::Int },
             Signature { params: vec![StaticType::Float, StaticType::Float], ret: StaticType::Float },
             Signature { params: vec![StaticType::Text, StaticType::Text], ret: StaticType::Text },
+            Signature { params: vec![StaticType::DateTime, StaticType::Int], ret: StaticType::DateTime },
         ]);
         self.register_native("+", plus_ty, |args| {
             if args.len() == 2 {
@@ -106,7 +126,8 @@ impl Environment {
                         let mut res = a.to_string();
                         res.push_str(b);
                         Value::Text(Rc::from(res))
-                    }
+                    },
+                    (Value::DateTime(ts), Value::Int(ms)) => Value::DateTime(ts + ms),
                     _ => Value::Void,
                 }
             } else {
@@ -124,6 +145,8 @@ impl Environment {
             Signature { params: vec![StaticType::Float, StaticType::Float], ret: StaticType::Float },
             Signature { params: vec![StaticType::Int], ret: StaticType::Int },
             Signature { params: vec![StaticType::Float], ret: StaticType::Float },
+            Signature { params: vec![StaticType::DateTime, StaticType::DateTime], ret: StaticType::Int },
+            Signature { params: vec![StaticType::DateTime, StaticType::Int], ret: StaticType::DateTime },
         ]);
         self.register_native("-", minus_ty, |args| {
             if args.is_empty() { return Value::Void; }
@@ -140,6 +163,8 @@ impl Environment {
                     (Value::Float(a), Value::Float(b)) => Value::Float(a - b),
                     (Value::Int(a), Value::Float(b)) => Value::Float(*a as f64 - b),
                     (Value::Float(a), Value::Int(b)) => Value::Float(a - *b as f64),
+                    (Value::DateTime(a), Value::DateTime(b)) => Value::Int(a - b),
+                    (Value::DateTime(a), Value::Int(b)) => Value::DateTime(a - b),
                     _ => Value::Void,
                 };
             }
@@ -192,6 +217,7 @@ impl Environment {
         let cmp_ty = StaticType::FunctionOverloads(vec![
             Signature { params: vec![StaticType::Int, StaticType::Int], ret: StaticType::Bool },
             Signature { params: vec![StaticType::Float, StaticType::Float], ret: StaticType::Bool },
+            Signature { params: vec![StaticType::DateTime, StaticType::DateTime], ret: StaticType::Bool },
         ]);
         self.register_native(">", cmp_ty.clone(), |args| {
              if args.len() != 2 { return Value::Void; }
@@ -200,6 +226,7 @@ impl Environment {
                  (Value::Int(a), Value::Float(b)) => Value::Bool((*a as f64) > *b),
                  (Value::Float(a), Value::Int(b)) => Value::Bool(*a > (*b as f64)),
                  (Value::Float(a), Value::Float(b)) => Value::Bool(a > b),
+                 (Value::DateTime(a), Value::DateTime(b)) => Value::Bool(a > b),
                  _ => Value::Bool(false),
              }
         });
@@ -211,6 +238,7 @@ impl Environment {
                  (Value::Int(a), Value::Float(b)) => Value::Bool((*a as f64) < *b),
                  (Value::Float(a), Value::Int(b)) => Value::Bool(*a < (*b as f64)),
                  (Value::Float(a), Value::Float(b)) => Value::Bool(a < b),
+                 (Value::DateTime(a), Value::DateTime(b)) => Value::Bool(a < b),
                  _ => Value::Bool(false),
              }
         });