[jsinterp] Handle Date at epoch 0

Closes #6400

test/test_youtube_signature.py

@@ -66,6 +66,10 @@ _SIG_TESTS = [
 ]
 
 _NSIG_TESTS = [
+    (
+        'https://www.youtube.com/s/player/7862ca1f/player_ias.vflset/en_US/base.js',
+        'X_LCxVDjAavgE5t', 'yxJ1dM6iz5ogUg',
+    ),
     (
         'https://www.youtube.com/s/player/9216d1f7/player_ias.vflset/en_US/base.js',
         'SLp9F5bwjAdhE9F-', 'gWnb9IK2DJ8Q1w',

yt_dlp/jsinterp.py

@@ -355,11 +355,11 @@ class JSInterpreter:
             obj = expr[4:]
             if obj.startswith('Date('):
                 left, right = self._separate_at_paren(obj[4:])
-                expr = unified_timestamp(
+                date = unified_timestamp(
                     self.interpret_expression(left, local_vars, allow_recursion), False)
-                if not expr:
+                if date is None:
                     raise self.Exception(f'Failed to parse date {left!r}', expr)
-                expr = self._dump(int(expr * 1000), local_vars) + right
+                expr = self._dump(int(date * 1000), local_vars) + right
             else:
                 raise self.Exception(f'Unsupported object {obj}', expr)