diff --git a/modules/lib/generators.nix b/modules/lib/generators.nix
index 795e485342..11b6abc07f 100644
--- a/modules/lib/generators.nix
+++ b/modules/lib/generators.nix
@@ -930,6 +930,10 @@ in
     preserved. YAML is written in block style with `---` and `...` document
     markers.
 
+    As of `jaq` 3.1.1, TOML date and time values are not supported. An existing
+    TOML file containing such values causes activation to fail without modifying
+    the file. Subsequent activations will also fail while those values remain.
+
     :::{.warning}
     This function is **experimental**: its interface and generated script may
     change without notice in future releases, as edge cases around the
diff --git a/tests/lib/generators/mkImpureConfigMerger.nix b/tests/lib/generators/mkImpureConfigMerger.nix
index 08e9443eb4..313756a6e9 100644
--- a/tests/lib/generators/mkImpureConfigMerger.nix
+++ b/tests/lib/generators/mkImpureConfigMerger.nix
@@ -102,6 +102,12 @@ let
       key: nix
     ...
   '';
+
+  userDateToml = builtins.toFile "user-date.toml" ''
+    # Keep this file intact when jaq cannot parse its date value.
+    date = 1979-05-27T07:32:00Z
+    userSetting = 5
+  '';
 in
 {
   nmt.script = ''
@@ -178,6 +184,15 @@ in
     $TMPDIR/merge-toml
     assertFileContent $tomlSettings ${expectedToml}
 
+    # jaq 3.1.1 cannot parse TOML dates. If a newer jaq can, they would pass
+    # through JSON as strings and come back as TOML strings, so this failing
+    # means the documented limitation needs revisiting.
+    cat ${userDateToml} > $tomlSettings
+    if $TMPDIR/merge-toml > /dev/null 2>&1; then
+      fail "Merging TOML with a date value must fail activation"
+    fi
+    assertFileContent $tomlSettings ${userDateToml}
+
     substitute ${mergeYaml} $TMPDIR/merge-yaml --subst-var TMPDIR
     chmod +x $TMPDIR/merge-yaml
     yamlSettings=$TMPDIR/hm-user/.config/testmerger/settings.yaml