diff --git a/src/slashing/evidence_verifier.rs b/src/slashing/evidence_verifier.rs
new file mode 100644
index 0000000..f180502
--- /dev/null
+++ b/src/slashing/evidence_verifier.rs
@@ -0,0 +1,82 @@
+// Evidence verifier utilities for slashing expiry checks
+
+pub type Slot = u64;
+pub type Epoch = u64;
+
+pub const SLOTS_PER_EPOCH: Slot = 32;
+pub const MAX_SLASHING_WINDOW: Slot = 8192; // in slots (~36 hours)
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct SlashingEvidence {
+    /// For single-slot infractions (double-vote) this contains the offending slot.
+    pub slot: Option<Slot>,
+    /// For surround-vote evidence include source and target epochs.
+    pub source_epoch: Option<Epoch>,
+    pub target_epoch: Option<Epoch>,
+}
+
+impl SlashingEvidence {
+    pub fn new(slot: Option<Slot>, source_epoch: Option<Epoch>, target_epoch: Option<Epoch>) -> Self {
+        Self { slot, source_epoch, target_epoch }
+    }
+}
+
+/// Returns inclusive slot range (start, end) that covers the infraction represented by `ev`.
+/// For epoch-based timestamps, we use the epoch's slot range: [epoch * SLOTS_PER_EPOCH, epoch * SLOTS_PER_EPOCH + (SLOTS_PER_EPOCH - 1)].
+/// If multiple timestamps present, the returned range spans the earliest start to the latest end.
+pub fn evidence_infraction_slot_range(ev: &SlashingEvidence) -> (Slot, Slot) {
+    // Start with large bounds depending on what is present
+    let mut starts: Vec<Slot> = Vec::new();
+    let mut ends: Vec<Slot> = Vec::new();
+
+    if let Some(s) = ev.slot {
+        starts.push(s);
+        ends.push(s);
+    }
+    if let Some(se) = ev.source_epoch {
+        let start = se.saturating_mul(SLOTS_PER_EPOCH);
+        let end = start + (SLOTS_PER_EPOCH - 1);
+        starts.push(start);
+        ends.push(end);
+    }
+    if let Some(te) = ev.target_epoch {
+        let start = te.saturating_mul(SLOTS_PER_EPOCH);
+        let end = start + (SLOTS_PER_EPOCH - 1);
+        starts.push(start);
+        ends.push(end);
+    }
+
+    // If nothing present, treat as slot 0
+    if starts.is_empty() {
+        return (0, 0);
+    }
+
+    let start = *starts.iter().min().unwrap();
+    let end = *ends.iter().max().unwrap();
+    (start, end)
+}
+
+/// Verify whether the evidence is still within the slashing window relative to `current_slot`.
+/// Returns true if the evidence is considered expired (outside the max slashing window).
+/// The rule: evidence is valid if earliest_infraction_start + MAX_SLASHING_WINDOW >= current_slot.
+/// Expired when earliest_start + MAX_SLASHING_WINDOW < current_slot.
+pub fn verify_evidence_expiry(ev: &SlashingEvidence, current_slot: Slot) -> bool {
+    let (earliest_start, _latest_end) = evidence_infraction_slot_range(ev);
+    // Accept evidence at the boundary (inclusive). Expire if strictly past the window.
+    let valid_until = earliest_start.saturating_add(MAX_SLASHING_WINDOW);
+    // expired = current_slot > valid_until
+    current_slot > valid_until
+}
+
+/// Verify surround vote semantics: evidence must include both source and target epochs and
+/// source_epoch < target_epoch. This function returns true if the surround evidence is valid
+/// and within the slashing window (not expired).
+pub fn verify_surround_vote(ev: &SlashingEvidence, current_slot: Slot) -> Result<bool, &'static str> {
+    match (ev.source_epoch, ev.target_epoch) {
+        (Some(s), Some(t)) => {
+            if s >= t { return Err("invalid_surround_vote_epochs"); }
+            Ok(!verify_evidence_expiry(ev, current_slot))
+        }
+        _ => Err("missing_surround_vote_epochs"),
+    }
+}
diff --git a/src/slashing/mod.rs b/src/slashing/mod.rs
index 0b967ca..c4e60d5 100644
--- a/src/slashing/mod.rs
+++ b/src/slashing/mod.rs
@@ -1,3 +1,4 @@
 pub mod cross_chain_relay;
 pub mod mempool;
 pub mod types;
+pub mod evidence_verifier;
diff --git a/tests/slashing/evidence_expiry_test.rs b/tests/slashing/evidence_expiry_test.rs
new file mode 100644
index 0000000..9169eed
--- /dev/null
+++ b/tests/slashing/evidence_expiry_test.rs
@@ -0,0 +1,40 @@
+use soroban_sdk::Env; // bring in crate for test context if needed
+
+use sorosusu_contracts::slashing::evidence_verifier::*;
+
+#[test]
+fn surround_vote_at_window_boundary_is_valid_and_one_past_is_expired() {
+    // earliest infraction at slot 1000 (from source_epoch)
+    let source_epoch = 1000 / SLOTS_PER_EPOCH;
+    let target_epoch = source_epoch + 1; // a surround that spans into next epoch
+    let ev = SlashingEvidence::new(None, Some(source_epoch), Some(target_epoch));
+
+    let earliest_start = evidence_infraction_slot_range(&ev).0;
+    let boundary_slot = earliest_start + MAX_SLASHING_WINDOW; // inclusive boundary
+
+    // At boundary: still valid
+    let expired_at_boundary = verify_evidence_expiry(&ev, boundary_slot);
+    assert_eq!(expired_at_boundary, false, "evidence at boundary should be valid");
+
+    // One slot past boundary: expired
+    let expired_one_past = verify_evidence_expiry(&ev, boundary_slot + 1);
+    assert_eq!(expired_one_past, true, "evidence one past boundary should be expired");
+}
+
+use proptest::prelude::*;
+
+proptest! {
+    #[test]
+    fn prop_evidence_window_symmetry(slot in 0u64..1_000_000u64, src_epoch in 0u64..1000u64, tgt_epoch in 0u64..1000u64, current_slot in 0u64..2_000_000u64) {
+        // Build evidence with random fields
+        let ev = SlashingEvidence::new(Some(slot), Some(src_epoch), Some(tgt_epoch));
+        let (start, _end) = evidence_infraction_slot_range(&ev);
+        let valid_until = start.saturating_add(MAX_SLASHING_WINDOW);
+
+        // Manual determination: not expired if current_slot <= valid_until
+        let manual_not_expired = current_slot <= valid_until;
+        let func_not_expired = !verify_evidence_expiry(&ev, current_slot);
+
+        prop_assert_eq!(manual_not_expired, func_not_expired);
+    }
+}