Address review comments: remove redundant casting and fix test cases

- Remove redundant TIMESTAMP_NS casting in _timediff_sql (lines 268-271)
  * Previously cast to TIMESTAMP_NS before calling _handle_nanosecond_diff
  * _handle_nanosecond_diff already handles casting via exp.cast (which avoids recasting)
  * Now passes expressions directly, matching _date_diff_sql pattern

- Fix TIMEDIFF test case to use valid Snowflake syntax
  * Changed from TIME literal '10:00:00.000000000'
  * To TIMESTAMP with CAST: CAST('2023-01-01 10:00:00.000000000' AS TIMESTAMP)
  * TIME literals don't work with TIMEDIFF in Snowflake

- Fix TIMEADD test case to use valid Snowflake syntax
  * Changed from TIME literal '10:00:00.000000000'
  * To TIMESTAMP with CAST: CAST('2023-01-01 10:00:00.000000000' AS TIMESTAMP)
  * TIME literals don't work with TIMEADD in Snowflake

Addresses review comments from VaggelisD on PR #6617

Author: fivetran-amrutabhimsenayachit

sqlglot/dialects/duckdb.py

@@ -144,11 +144,16 @@ def _last_day_sql(self: DuckDB.Generator, expression: exp.LastDay) -> str:
 
 
 def _unwrap_cast(expr: exp.Expression) -> exp.Expression:
-    """Unwrap Cast expression to avoid double-casting that loses nanosecond precision.
+    """Unwrap Cast expression to avoid nested casts when recasting to different types.
 
-    Nested casts can lose precision when converting between
-    timestamp types. By unwrapping the inner cast, we go directly from the source
-    expression to TIMESTAMP_NS, preserving nanosecond precision.
+    While exp.cast avoids recasting to the SAME type, it doesn't unwrap casts to
+    DIFFERENT types. This helper extracts the inner expression before casting to
+    avoid nested casts like CAST(CAST(x AS TIMESTAMP) AS TIMESTAMP_NS).
+
+    Example:
+        Input: CAST('2023-01-01' AS TIMESTAMP)
+        Without unwrap: CAST(CAST('2023-01-01' AS TIMESTAMP) AS TIMESTAMP_NS)
+        With unwrap: CAST('2023-01-01' AS TIMESTAMP_NS)
     """
     return expr.this if isinstance(expr, exp.Cast) else expr
 
@@ -266,9 +271,7 @@ def _timediff_sql(self: DuckDB.Generator, expression: exp.TimeDiff) -> str:
     unit = expression.args.get("unit")
 
     if _is_nanosecond_unit(unit):
-        this_ts = exp.cast(expression.this, exp.DataType.Type.TIMESTAMP_NS)
-        expr_ts = exp.cast(expression.expression, exp.DataType.Type.TIMESTAMP_NS)
-        return _handle_nanosecond_diff(self, expr_ts, this_ts)
+        return _handle_nanosecond_diff(self, expression.expression, expression.this)
 
     this = exp.cast(expression.this, exp.DataType.Type.TIME)
     expr = exp.cast(expression.expression, exp.DataType.Type.TIME)

tests/dialects/test_snowflake.py

@@ -2493,10 +2493,10 @@ def test_timestamps(self):
 
         # Test TIMEDIFF with NANOSECOND - DuckDB uses EPOCH_NS for nanosecond precision
         self.validate_all(
-            "TIMEDIFF(NANOSECOND, '10:00:00.000000000', '10:00:00.123456789')",
+            "TIMEDIFF(NANOSECOND, CAST('2023-01-01 10:00:00.000000000' AS TIMESTAMP), CAST('2023-01-01 10:00:00.123456789' AS TIMESTAMP))",
             write={
-                "duckdb": "EPOCH_NS(CAST('10:00:00.123456789' AS TIMESTAMP_NS)) - EPOCH_NS(CAST('10:00:00.000000000' AS TIMESTAMP_NS))",
-                "snowflake": "DATEDIFF(NANOSECOND, '10:00:00.000000000', '10:00:00.123456789')",
+                "duckdb": "EPOCH_NS(CAST('2023-01-01 10:00:00.123456789' AS TIMESTAMP_NS)) - EPOCH_NS(CAST('2023-01-01 10:00:00.000000000' AS TIMESTAMP_NS))",
+                "snowflake": "DATEDIFF(NANOSECOND, CAST('2023-01-01 10:00:00.000000000' AS TIMESTAMP), CAST('2023-01-01 10:00:00.123456789' AS TIMESTAMP))",
             },
         )
 
@@ -2518,10 +2518,10 @@ def test_timestamps(self):
 
         # Test TIMEADD with NANOSECOND - DuckDB uses MAKE_TIMESTAMP_NS
         self.validate_all(
-            "TIMEADD(NANOSECOND, 123456789, '10:00:00.000000000')",
+            "TIMEADD(NANOSECOND, 123456789, CAST('2023-01-01 10:00:00.000000000' AS TIMESTAMP))",
             write={
-                "duckdb": "MAKE_TIMESTAMP_NS(EPOCH_NS(CAST('10:00:00.000000000' AS TIMESTAMP_NS)) + 123456789)",
-                "snowflake": "TIMEADD(NANOSECOND, 123456789, '10:00:00.000000000')",
+                "duckdb": "MAKE_TIMESTAMP_NS(EPOCH_NS(CAST('2023-01-01 10:00:00.000000000' AS TIMESTAMP_NS)) + 123456789)",
+                "snowflake": "TIMEADD(NANOSECOND, 123456789, CAST('2023-01-01 10:00:00.000000000' AS TIMESTAMP))",
             },
         )