Migrate codebase across to new SlotClock API

eth2/utils/slot_clock/src/lib.rs

@@ -5,7 +5,7 @@ mod metrics;
 mod system_time_slot_clock;
 mod testing_slot_clock;
 
-use std::time::{Duration, Instant};
+use std::time::{Duration, Instant, SystemTime, UNIX_EPOCH};
 
 pub use crate::system_time_slot_clock::SystemTimeSlotClock;
 pub use crate::testing_slot_clock::TestingSlotClock;
@@ -13,6 +13,24 @@ pub use metrics::scrape_for_metrics;
 pub use types::Slot;
 
 pub trait SlotClock: Send + Sync + Sized {
+    fn from_eth2_genesis(
+        genesis_slot: Slot,
+        genesis_seconds: u64,
+        slot_duration: Duration,
+    ) -> Option<Self> {
+        let duration_between_now_and_unix_epoch =
+            SystemTime::now().duration_since(UNIX_EPOCH).ok()?;
+        let duration_between_unix_epoch_and_genesis = Duration::from_secs(genesis_seconds);
+
+        if duration_between_now_and_unix_epoch < duration_between_unix_epoch_and_genesis {
+            None
+        } else {
+            let genesis_instant = Instant::now()
+                - (duration_between_now_and_unix_epoch - duration_between_unix_epoch_and_genesis);
+            Some(Self::new(genesis_slot, genesis_instant, slot_duration))
+        }
+    }
+
     fn new(genesis_slot: Slot, genesis: Instant, slot_duration: Duration) -> Self;
 
     fn present_slot(&self) -> Option<Slot>;