diff --git a/src/constants.rs b/src/constants.rs
index fe0938be..cbc306ad 100644
--- a/src/constants.rs
+++ b/src/constants.rs
@@ -1,7 +1,8 @@
 //! Named constants shared across the contract.
 //!
 //! Hoisted here so magic numbers (`9999`, `10000`, `86400`) appear exactly
-//! once in the codebase (#482).
+//! once in the codebase (#482). All callers import these constants rather
+//! than using local literals (#671).
 
 /// Basis-point denominator: 10_000 bps == 100%.
 pub(crate) const BPS_DENOMINATOR: u32 = 10_000;
diff --git a/src/tests/test_lifecycle.rs b/src/tests/test_lifecycle.rs
index aa831a23..edf843f2 100644
--- a/src/tests/test_lifecycle.rs
+++ b/src/tests/test_lifecycle.rs
@@ -1,5 +1,5 @@
 use super::helpers::*;
-use crate::{Category, VotingKey, SECONDS_PER_DAY};
+use crate::{lifecycle::calculate_deadline, Category, Error, VotingKey, SECONDS_PER_DAY};
 use soroban_sdk::{FromVal, String, TryFromVal};
 
 // ── lifecycle events ────────────────────────────────────────────────────────────
@@ -430,3 +430,31 @@ fn test_multi_step_sequence() {
     let id = client.create_campaign(&params);
     client.cancel_campaign(&id);
 }
+
+// ── calculate_deadline ───────────────────────────────────────────────────────────
+
+#[test]
+fn test_calculate_deadline_happy_path() {
+    let current_time = 1_000_000;
+    let duration_days = 30;
+    let expected = current_time + duration_days * SECONDS_PER_DAY;
+    assert_eq!(
+        calculate_deadline(current_time, duration_days).unwrap(),
+        expected
+    );
+}
+
+#[test]
+fn test_calculate_deadline_zero_days() {
+    let current_time = 1_000_000;
+    assert_eq!(calculate_deadline(current_time, 0).unwrap(), current_time);
+}
+
+#[test]
+fn test_calculate_deadline_overflow_rejected() {
+    let huge_days = u64::MAX / SECONDS_PER_DAY + 1;
+    assert_eq!(
+        calculate_deadline(0, huge_days),
+        Err(Error::ValidationFailed)
+    );
+}