diff --git a/routes/operations.js b/routes/operations.js
index 7ff76c3..7566c4a 100644
--- a/routes/operations.js
+++ b/routes/operations.js
@@ -734,6 +734,21 @@ module.exports = function(pool) {
             const scaleFactor = (scale != null) ? parseFloat(scale) : 1.0;
             const dateOffset  = (date_offset || '0 days').trim() || '0 days';
 
+            // The offset is interpolated into the SQL as an interval literal, so a
+            // typo would surface as a Postgres parse error mid-statement. Ask
+            // Postgres to parse it on its own first, where the failure is cheap and
+            // can be reported against the field the user typed it into. Negative
+            // intervals are fine and useful -- '-90 days' pulls a plan back a
+            // quarter -- so this checks validity, not sign.
+            try {
+                await pool.query(`SELECT $1::interval`, [dateOffset]);
+            } catch {
+                return res.status(400).json({
+                    error: `"${dateOffset}" is not a valid interval. Try something like `
+                         + `"12 months", "-90 days" or "0 days".`
+                });
+            }
+
             // Cloning from a named segment is how a period with no baseline gets a
             // shape: pick the ledger line -- prior year, plan -- and copy its mix
             // forward. That means reading rows exclude_iters normally keeps
diff --git a/ui/src/components/OperationPanel.jsx b/ui/src/components/OperationPanel.jsx
index ba622da..ce17a72 100644
--- a/ui/src/components/OperationPanel.jsx
+++ b/ui/src/components/OperationPanel.jsx
@@ -817,6 +817,8 @@ export default function OperationPanel({
                                <datalist id="pf-clone-offsets">
                                  <option value="12 months" />
                                  <option value="24 months" />
+                                 <option value="-90 days" />
+                                 <option value="-12 months" />
                                  <option value="0 days" />
                                </datalist>
                              </div>