diff --git a/crates/vm/src/stdlib/time.rs b/crates/vm/src/stdlib/time.rs
index 02288ca45..b645c1ab7 100644
--- a/crates/vm/src/stdlib/time.rs
+++ b/crates/vm/src/stdlib/time.rs
@@ -19,7 +19,7 @@ mod decl {
     use crate::convert::{ToPyException, ToPyObject};
     use crate::{
         AsObject, Py, PyObjectRef, PyResult, VirtualMachine,
-        builtins::{PyStr, PyStrRef, PyTypeRef},
+        builtins::{PyFloat, PyStr, PyStrRef, PyTypeRef},
         class::PyClassDef,
         common::wtf8::Wtf8Buf,
         function::{Either, FuncArgs, OptionalArg, OptionalOption},
@@ -143,7 +143,11 @@ mod decl {
                 .and_then(|seconds| seconds.checked_mul(SEC_TO_NS))
                 .ok_or_else(overflow)?
         } else {
-            let seconds = object.try_float(vm)?.to_f64();
+            // PyFloat_AsDouble reads float subclasses without calling __float__.
+            let seconds = match object.downcast_ref::<PyFloat>() {
+                Some(value) => value.to_f64(),
+                None => object.try_float(vm)?.to_f64(),
+            };
             if seconds.is_nan() {
                 return Err(vm.new_value_error("Invalid value NaN (not a number)"));
             }