diff --git a/routes/sources.js b/routes/sources.js
index 6376264..f09d77e 100644
--- a/routes/sources.js
+++ b/routes/sources.js
@@ -280,6 +280,22 @@ module.exports = function(pool) {
     });
 
     // deregister a source — does not drop existing forecast tables
+    router.get('/dim-period/cols', async (req, res) => {
+        try {
+            const result = await pool.query(`
+                SELECT column_name
+                FROM information_schema.columns
+                WHERE table_schema = 'pf' AND table_name = 'dim_period'
+                  AND column_name NOT IN ('sdat', 'edat', 'drange', 'ndays')
+                ORDER BY ordinal_position
+            `);
+            res.json(result.rows.map(r => r.column_name));
+        } catch (err) {
+            console.error(err);
+            res.status(500).json({ error: err.message });
+        }
+    });
+
     router.delete('/sources/:id', async (req, res) => {
         try {
             const result = await pool.query(
diff --git a/ui/src/views/Setup.jsx b/ui/src/views/Setup.jsx
index 52480ae..c60273e 100644
--- a/ui/src/views/Setup.jsx
+++ b/ui/src/views/Setup.jsx
@@ -24,9 +24,11 @@ export default function Setup({ refreshSources }) {
   const [saving,         setSaving]         = useState(false)
   const [generating,     setGenerating]     = useState(false)
   const [msg,            setMsg]            = useState(null)
+  const [dimPeriodCols,  setDimPeriodCols]  = useState([])
 
   useEffect(() => {
     fetch('/api/tables').then(r => r.json()).then(setTables).catch(console.error)
+    fetch('/api/dim-period/cols').then(r => r.json()).then(setDimPeriodCols).catch(console.error)
     loadSources()
   }, [])
 
@@ -324,14 +326,15 @@ export default function Setup({ refreshSources }) {
                           />
                         </td>
                         <td className="px-3 py-1.5">
-                          <input
-                            type="text"
+                          <select
                             value={col.dim_period_col || ''}
                             onChange={e => updateCol(i, 'dim_period_col', e.target.value || null)}
-                            placeholder="—"
                             disabled={col.role !== 'dimension' || !col.dim_group}
-                            className="border border-transparent hover:border-gray-200 focus:border-gray-300 rounded px-1.5 py-0.5 w-full outline-none bg-transparent disabled:opacity-20 disabled:cursor-default font-mono text-xs"
-                          />
+                            className="text-xs px-1.5 py-0.5 rounded border border-transparent hover:border-gray-200 focus:border-gray-300 outline-none bg-transparent disabled:opacity-20 disabled:cursor-default font-mono w-full"
+                          >
+                            <option value="">—</option>
+                            {dimPeriodCols.map(c => <option key={c} value={c}>{c}</option>)}
+                          </select>
                         </td>
                         <td className="px-3 py-1.5">
                           <input