diff --git a/apps/server/src/routes/tariffs.ts b/apps/server/src/routes/tariffs.ts
index 8b7b433..d43947e 100644
--- a/apps/server/src/routes/tariffs.ts
+++ b/apps/server/src/routes/tariffs.ts
@@ -62,12 +62,37 @@ export async function tariffRoutes(app: FastifyInstance, db: Db): Promise<void>
       if (problems.length) {
         return reply.code(400).send({ error: "invalid tariff structure", problems });
       }
+
+      // effectiveFrom must NOT be in the past. A version is selected by
+      // "latest effectiveFrom <= entry time", so a backdated effectiveFrom would
+      // retroactively reprice already-entered sessions — exactly the immutability
+      // the versioning exists to prevent (wiki/concepts/tariff.md). So we forbid
+      // backdating: a new version applies only from publish (now) forward; a future
+      // effectiveFrom (scheduling a price change) is allowed. A small skew tolerance
+      // absorbs client/server clock drift + request round-trip. Once a car has
+      // entered, no later publish can reprice it (no effectiveFrom can predate it).
+      const now = Date.now();
+      const SKEW_MS = 60_000; // 1 min: clock skew + round-trip slack
+      let effective = new Date().toISOString();
+      if (effectiveFrom != null) {
+        const t = Date.parse(effectiveFrom);
+        if (Number.isNaN(t)) {
+          return reply.code(400).send({ error: "effectiveFrom must be a valid ISO-8601 timestamp" });
+        }
+        if (t < now - SKEW_MS) {
+          return reply.code(400).send({
+            error: "effectiveFrom cannot be in the past — backdating a tariff would retroactively reprice entered sessions",
+          });
+        }
+        effective = new Date(t).toISOString();
+      }
+
       const tariffId = ensureSiteTariff();
       const id = randomUUID();
       const row = {
         id,
         tariffId,
-        effectiveFrom: effectiveFrom ?? new Date().toISOString(),
+        effectiveFrom: effective,
         currency,
         structure: structure as unknown as Record<string, unknown>,
         createdBy: req.user?.username ?? null,