diff --git a/posthog/hogql/functions/mapping.py b/posthog/hogql/functions/mapping.py
index cc7124adca7ce..08bee305b933e 100644
--- a/posthog/hogql/functions/mapping.py
+++ b/posthog/hogql/functions/mapping.py
@@ -2,6 +2,7 @@
 from itertools import chain
 from typing import Optional
 
+
 from posthog.cloud_utils import is_cloud, is_ci
 from posthog.hogql import ast
 from posthog.hogql.ast import (
@@ -80,16 +81,13 @@ class HogQLFunctionMeta:
 
 
 def compare_types(arg_types: list[ConstantType], sig_arg_types: tuple[ConstantType, ...]):
-    _sig_arg_types = list(sig_arg_types)
     if len(arg_types) != len(sig_arg_types):
         return False
 
-    for index, arg_type in enumerate(arg_types):
-        _sig_arg_type = _sig_arg_types[index]
-        if not isinstance(arg_type, _sig_arg_type.__class__):
-            return False
-
-    return True
+    return all(
+        isinstance(sig_arg_type, UnknownType) or isinstance(arg_type, sig_arg_type.__class__)
+        for arg_type, sig_arg_type in zip(arg_types, sig_arg_types)
+    )
 
 
 HOGQL_COMPARISON_MAPPING: dict[str, ast.CompareOperationOp] = {
@@ -448,15 +446,59 @@ def compare_types(arg_types: list[ConstantType], sig_arg_types: tuple[ConstantTy
     "toStartOfYear": HogQLFunctionMeta("toStartOfYear", 1, 1),
     "toStartOfISOYear": HogQLFunctionMeta("toStartOfISOYear", 1, 1),
     "toStartOfQuarter": HogQLFunctionMeta("toStartOfQuarter", 1, 1),
-    "toStartOfMonth": HogQLFunctionMeta("toStartOfMonth", 1, 1),
+    "toStartOfMonth": HogQLFunctionMeta(
+        "toStartOfMonth",
+        1,
+        1,
+        signatures=[
+            ((UnknownType(),), DateType()),
+        ],
+    ),
     "toLastDayOfMonth": HogQLFunctionMeta("toLastDayOfMonth", 1, 1),
     "toMonday": HogQLFunctionMeta("toMonday", 1, 1),
-    "toStartOfWeek": HogQLFunctionMeta("toStartOfWeek", 1, 2),
-    "toStartOfDay": HogQLFunctionMeta("toStartOfDay", 1, 2),
+    "toStartOfWeek": HogQLFunctionMeta(
+        "toStartOfWeek",
+        1,
+        2,
+        signatures=[
+            ((UnknownType(),), DateType()),
+            ((UnknownType(), UnknownType()), DateType()),
+        ],
+    ),
+    "toStartOfDay": HogQLFunctionMeta(
+        "toStartOfDay",
+        1,
+        2,
+        signatures=[
+            ((UnknownType(),), DateTimeType()),
+            ((UnknownType(), UnknownType()), DateTimeType()),
+        ],
+    ),
     "toLastDayOfWeek": HogQLFunctionMeta("toLastDayOfWeek", 1, 2),
-    "toStartOfHour": HogQLFunctionMeta("toStartOfHour", 1, 1),
-    "toStartOfMinute": HogQLFunctionMeta("toStartOfMinute", 1, 1),
-    "toStartOfSecond": HogQLFunctionMeta("toStartOfSecond", 1, 1),
+    "toStartOfHour": HogQLFunctionMeta(
+        "toStartOfHour",
+        1,
+        1,
+        signatures=[
+            ((UnknownType(),), DateTimeType()),
+        ],
+    ),
+    "toStartOfMinute": HogQLFunctionMeta(
+        "toStartOfMinute",
+        1,
+        1,
+        signatures=[
+            ((UnknownType(),), DateTimeType()),
+        ],
+    ),
+    "toStartOfSecond": HogQLFunctionMeta(
+        "toStartOfSecond",
+        1,
+        1,
+        signatures=[
+            ((UnknownType(),), DateTimeType()),
+        ],
+    ),
     "toStartOfFiveMinutes": HogQLFunctionMeta("toStartOfFiveMinutes", 1, 1),
     "toStartOfTenMinutes": HogQLFunctionMeta("toStartOfTenMinutes", 1, 1),
     "toStartOfFifteenMinutes": HogQLFunctionMeta("toStartOfFifteenMinutes", 1, 1),
@@ -472,7 +514,17 @@ def compare_types(arg_types: list[ConstantType], sig_arg_types: tuple[ConstantTy
     "dateSub": HogQLFunctionMeta("dateSub", 3, 3),
     "timeStampAdd": HogQLFunctionMeta("timeStampAdd", 2, 2),
     "timeStampSub": HogQLFunctionMeta("timeStampSub", 2, 2),
-    "now": HogQLFunctionMeta("now64", 0, 1, tz_aware=True, case_sensitive=False),
+    "now": HogQLFunctionMeta(
+        "now64",
+        0,
+        1,
+        tz_aware=True,
+        case_sensitive=False,
+        signatures=[
+            ((), DateTimeType()),
+            ((UnknownType(),), DateTimeType()),
+        ],
+    ),
     "nowInBlock": HogQLFunctionMeta("nowInBlock", 1, 1),
     "rowNumberInBlock": HogQLFunctionMeta("rowNumberInBlock", 0, 0),
     "rowNumberInAllBlocks": HogQLFunctionMeta("rowNumberInAllBlocks", 0, 0),
diff --git a/posthog/hogql/test/test_mapping.py b/posthog/hogql/test/test_mapping.py
index 6e074687d22cf..8dc805dace346 100644
--- a/posthog/hogql/test/test_mapping.py
+++ b/posthog/hogql/test/test_mapping.py
@@ -1,4 +1,8 @@
-from posthog.hogql.ast import FloatType, IntegerType
+from posthog.hogql.ast import FloatType, IntegerType, DateType
+from posthog.hogql.base import UnknownType
+from posthog.hogql.context import HogQLContext
+from posthog.hogql.parser import parse_expr
+from posthog.hogql.printer import print_ast
 from posthog.test.base import BaseTest
 from typing import Optional
 from posthog.hogql.functions.mapping import (
@@ -7,6 +11,7 @@
     find_hogql_aggregation,
     find_hogql_posthog_function,
     HogQLFunctionMeta,
+    HOGQL_CLICKHOUSE_FUNCTIONS,
 )
 
 
@@ -60,3 +65,26 @@ def test_compare_types_mismatch_lengths(self):
     def test_compare_types_mismatch_differing_order(self):
         res = compare_types([IntegerType(), FloatType()], (FloatType(), IntegerType()))
         assert res is False
+
+    def test_unknown_type_mapping(self):
+        HOGQL_CLICKHOUSE_FUNCTIONS["overloadedFunction"] = HogQLFunctionMeta(
+            "overloadFailure",
+            1,
+            1,
+            overloads=[((DateType,), "overloadSuccess")],
+        )
+
+        HOGQL_CLICKHOUSE_FUNCTIONS["dateEmittingFunction"] = HogQLFunctionMeta(
+            "dateEmittingFunction",
+            1,
+            1,
+            signatures=[
+                ((UnknownType(),), DateType()),
+            ],
+        )
+        ast = print_ast(
+            parse_expr("overloadedFunction(dateEmittingFunction('123123'))"),
+            HogQLContext(self.team.pk, enable_select_queries=True),
+            "clickhouse",
+        )
+        assert "overloadSuccess" in ast