diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,10 +1,12 @@
 # Revision history of io-sim
 
-## next release
-
-### Breaking changes
+## 1.9.1.0
 
 ### Non-breaking changes
+
+* Reverted rounding of `si-timers` API to microsecond to match `IO` behaviour.
+  It may cause surprising issues downstream when time is not advancing as
+  expected because of rounding to `0`.
 
 ## 1.9.0.0
 
diff --git a/io-sim.cabal b/io-sim.cabal
--- a/io-sim.cabal
+++ b/io-sim.cabal
@@ -1,6 +1,6 @@
 cabal-version:       3.4
 name:                io-sim
-version:             1.9.0.0
+version:             1.9.1.0
 synopsis:            A pure simulator for monadic concurrency with STM.
 description:
   A pure simulator monad with support of concurrency (base & async style), stm,
diff --git a/src/Control/Monad/IOSim/Types.hs b/src/Control/Monad/IOSim/Types.hs
--- a/src/Control/Monad/IOSim/Types.hs
+++ b/src/Control/Monad/IOSim/Types.hs
@@ -725,8 +725,7 @@
 
 instance SI.MonadDelay (IOSim s) where
   threadDelay d =
-    IOSim $ oneShot $ \k -> ThreadDelay (SI.roundDiffTimeToMicroseconds d)
-                                        (k ())
+    IOSim $ oneShot $ \k -> ThreadDelay d (k ())
 
 data Timeout s = Timeout !(TVar s TimeoutState) !TimeoutId
                -- ^ a timeout
@@ -766,15 +765,11 @@
   timeout d action
     | d <  0 = Just <$> action
     | d == 0 = return Nothing
-    | otherwise = IOSim $ oneShot $ \k ->
-                          StartTimeout (SI.roundDiffTimeToMicroseconds d)
-                                       (runIOSim action)
-                                       k
+    | otherwise = IOSim $ oneShot $ \k -> StartTimeout d (runIOSim action) k
 
-  registerDelay d = IOSim $ oneShot $ \k ->
-    RegisterDelay (SI.roundDiffTimeToMicroseconds d) k
+  registerDelay d = IOSim $ oneShot $ \k -> RegisterDelay d k
   registerDelayCancellable d = do
-    t <- newTimeout (SI.roundDiffTimeToMicroseconds d)
+    t <- newTimeout d
     return (readTimeout t, cancelTimeout t)
 
 newtype TimeoutException = TimeoutException TimeoutId deriving Eq