feat: gix-date Time local_time helper and Sub implementation

Author: dvtkrlbs

gix-date/src/time/mod.rs

@@ -65,6 +65,7 @@ mod sign {
 }
 
 mod impls {
+    use std::ops::Sub;
     use crate::{time::Sign, Time};
 
     impl Default for Time {
@@ -76,4 +77,18 @@ mod impls {
             }
         }
     }
+
+    impl Sub for Time {
+        type Output = i64;
+
+        fn sub(self, rhs: Self) -> Self::Output {
+            self.local_time() - rhs.local_time()
+        }
+    }
+
+    impl Time {
+        fn local_time(&self) -> i64 {
+            self.seconds + self.offset as i64
+        }
+    }
 }