diff --git a/candidate.go b/candidate.go
index 52f255c..64fd6ad 100644
--- a/candidate.go
+++ b/candidate.go
@@ -142,6 +142,16 @@ func (c *Candidate) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	if time.Since(v.VoteSent).Abs() > 15*time.Second {
+		http.Error(
+			w,
+			fmt.Sprintf("excessive time difference (%.1f seconds); delay, replay, or clock skew", time.Since(v.VoteSent).Seconds()),
+			http.StatusBadRequest,
+		)
+	}
+
+	c.resp.ResponseSent = time.Now().UTC()
+
 	js = lo.Must(json.Marshal(c.resp))
 
 	w.Header().Set("Content-Type", "application/json")
diff --git a/voter.go b/voter.go
index 3f995fb..d4a98c3 100644
--- a/voter.go
+++ b/voter.go
@@ -21,18 +21,18 @@ type Voter struct {
 }
 
 type vote struct {
-	VoterID             string `json:"voterID"`
-	LastSeenCandidateID string `json:"lastSeenCandidateID"`
-	NumPollsSinceChange int    `json:"numPollsSinceChange"`
-	// TODO: Add timestamp
+	VoterID             string    `json:"voterID"`
+	LastSeenCandidateID string    `json:"lastSeenCandidateID"`
+	NumPollsSinceChange int       `json:"numPollsSinceChange"`
+	VoteSent            time.Time `json:"voteSent"`
 
 	// Used internally by Candidate
 	received time.Time
 }
 
 type voteResponse struct {
-	CandidateID string `json:"candidateID"`
-	// TODO: Add timestamp
+	CandidateID  string    `json:"candidateID"`
+	ResponseSent time.Time `json:"responseSent"`
 }
 
 func NewVoter(url string, signingKey string) *Voter {
@@ -105,6 +105,8 @@ func (v *Voter) poll(update <-chan time.Duration, t *time.Ticker) bool {
 }
 
 func (v *Voter) sendVote() {
+	v.vote.VoteSent = time.Now().UTC()
+
 	for _, c := range v.candidates {
 		c.voteIfNo(&v.vote)
 	}
@@ -152,6 +154,10 @@ func (v *Voter) sendVote() {
 		return
 	}
 
+	if time.Since(vr.ResponseSent).Abs() > 15*time.Second {
+		v.log("excessive time difference (%.1f seconds); delay, replay, or clock skew", time.Since(vr.ResponseSent).Seconds())
+	}
+
 	if vr.CandidateID == v.vote.LastSeenCandidateID {
 		v.vote.NumPollsSinceChange++
 	} else {