diff --git a/src/components/ActivityForm.tsx b/src/components/ActivityForm.tsx
index a3b838e..58e1969 100644
--- a/src/components/ActivityForm.tsx
+++ b/src/components/ActivityForm.tsx
@@ -266,13 +266,21 @@ function FieldControl({
             onChange={onChange}
           />
         );
+      // A `datetime-local` input yields "2026-08-17T18:31" — no seconds, no
+      // timezone. The server validates this field as a timestamp and rejects
+      // that shape outright:
+      //     validation failed for activity hdfc-act-disburse: disbursed_at(type)
+      // So the browser value is converted to RFC3339 UTC on the way out, and
+      // back to local wall-clock on the way in. Storing the ISO string and
+      // slicing it for display would show the UTC time in the picker — 13:01
+      // for an 18:31 selection — which looks like the control losing input.
       case 'datetime':
         return (
           <Input
             type="datetime-local"
             disabled={readOnly}
-            value={String(value ?? '').slice(0, 16)}
-            onChange={onChange}
+            value={isoToLocalInput(value)}
+            onChange={(v) => onChange(localInputToIso(v))}
           />
         );
       case 'longtext':
@@ -412,3 +420,28 @@ function OcrUpload({
     </div>
   );
 }
+
+/**
+ * `datetime-local` <-> RFC3339, the two conversions the datetime control needs.
+ *
+ * The input speaks local wall-clock with minute precision and no zone; the
+ * workflow's timestamp validator wants a full RFC3339 instant. Neither will
+ * accept the other's format, and getting it wrong fails as an opaque
+ * "<field>(type)" validation error with no hint about which half is at fault.
+ */
+function localInputToIso(v: string): string {
+  if (!v) return '';
+  const d = new Date(v); // parsed as LOCAL time, which is what the picker meant
+  return Number.isNaN(d.getTime()) ? v : d.toISOString();
+}
+
+function isoToLocalInput(v: unknown): string {
+  const s = String(v ?? '');
+  if (!s) return '';
+  const d = new Date(s);
+  // Not a parseable instant — hand it back untouched rather than blanking the
+  // field, so a value the server sent stays visible even if we misread it.
+  if (Number.isNaN(d.getTime())) return s.slice(0, 16);
+  const p = (n: number) => String(n).padStart(2, '0');
+  return `${d.getFullYear()}-${p(d.getMonth() + 1)}-${p(d.getDate())}T${p(d.getHours())}:${p(d.getMinutes())}`;
+}