Calculate Time.time_since_startup on update()

As requested in #3264.

crates/bevy_core/src/time/time.rs

@@ -9,6 +9,7 @@ pub struct Time {
     delta_seconds_f64: f64,
     delta_seconds: f32,
     seconds_since_startup: f64,
+    time_since_startup: Duration,
     startup: Instant,
 }
 
@@ -20,6 +21,7 @@ impl Default for Time {
             startup: Instant::now(),
             delta_seconds_f64: 0.0,
             seconds_since_startup: 0.0,
+            time_since_startup: Duration::from_secs(0),
             delta_seconds: 0.0,
         }
     }
@@ -38,8 +40,8 @@ impl Time {
             self.delta_seconds = self.delta.as_secs_f32();
         }
 
-        let duration_since_startup = instant - self.startup;
-        self.seconds_since_startup = duration_since_startup.as_secs_f64();
+        self.time_since_startup = instant - self.startup;
+        self.seconds_since_startup = self.time_since_startup.as_secs_f64();
         self.last_update = Some(instant);
     }
 
@@ -81,7 +83,7 @@ impl Time {
 
     /// The ['Duration'] from startup to the last update
     pub fn time_since_startup(&self) -> Duration {
-        self.last_update.unwrap_or(self.startup) - self.startup
+        self.time_since_startup
     }
 }