Add an utility function to get the first timestamp of a slot

prdoc/pr_5316.prdoc

@@ -0,0 +1,11 @@
+title: add timestamp function to sp-consensus-slots
+
+doc:
+  - audience: Node Dev
+    description: |
+      Added timestamp function to sp-consensus-slots to get the first timestamp
+      of the given slot.
+
+crates:
+  - name: sp-consensus-slots
+    bump: minor

substrate/primitives/consensus/slots/src/lib.rs

@@ -91,6 +91,13 @@ impl Slot {
 		Slot(timestamp.as_millis() / slot_duration.as_millis())
 	}
 
+	/// Timestamp of the start of the slot.
+	///
+	/// Returns `None` if would overflow for given `SlotDuration`.
+	pub fn timestamp(&self, slot_duration: SlotDuration) -> Option<Timestamp> {
+		slot_duration.as_millis().checked_mul(self.0).map(Timestamp::new)
+	}
+
 	/// Saturating addition.
 	pub fn saturating_add<T: Into<u64>>(self, rhs: T) -> Self {
 		Self(self.0.saturating_add(rhs.into()))