fix: address @kanitw's concerns

src/compile/format.ts

@@ -79,7 +79,12 @@ export function formatSignalRef({
         });
       }
     }
-    if (type === 'temporal' && config.timeFormatType) {
+    if (
+      type === 'temporal' &&
+      config.timeFormatType &&
+      isFieldDef(fieldOrDatumDef) &&
+      fieldOrDatumDef.timeUnit === undefined
+    ) {
       return formatCustomType({
         fieldOrDatumDef,
         format: config.timeFormat,

src/config.ts

@@ -193,7 +193,7 @@ export interface VLOnlyConfig<ES extends ExprRef | SignalRef> {
    * [Custom format type](https://vega.github.io/vega-lite/docs/config.html#custom-format-type)
    * for `config.timeFormat`.
    *
-   * __Default value:__ `undefined` -- This is equilvalent to call D3-format, which is exposed as [`format` in Vega-Expression](https://vega.github.io/vega/docs/expressions/#format).
+   * __Default value:__ `undefined` -- This is equilvalent to call D3-time-format, which is exposed as [`timeFormat` in Vega-Expression](https://vega.github.io/vega/docs/expressions/#timeFormat).
    * __Note:__ You must also set `customFormatTypes` to `true` and there must *not* be a `timeUnit` defined to use this feature.
    */
   timeFormatType?: string;

test/compile/format.test.ts

@@ -231,14 +231,14 @@ describe('Format', () => {
     it('should use a custom formatter datumDef if config.timeFormatType is present', () => {
       expect(
         formatSignalRef({
-          fieldOrDatumDef: {datum: 200, type: 'temporal'},
+          fieldOrDatumDef: {field: 'date', type: 'temporal'},
           format: undefined,
           formatType: undefined,
           expr: 'parent',
           config: {timeFormat: 'abc', timeFormatType: 'customFormatter', customFormatTypes: true}
         })
       ).toEqual({
-        signal: 'customFormatter(200, "abc")'
+        signal: 'customFormatter(parent["date"], "abc")'
       });
     });