diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,21 @@
 o'clock uses [PVP Versioning][1].
 The change log is available [on GitHub][2].
 
+1.0.0
+=====
+
+* TODO
+
+0.1.1
+=====
+
+* [#98](https://github.com/serokell/o-clock/issues/98):
+  Support GHC-8.0.2.
+* [#95](https://github.com/serokell/o-clock/issues/95):
+  Add `Semigroup` and `Monoid` instances for `Time`.
+* [#93](https://github.com/serokell/o-clock/issues/93):
+  Remove `transformers` dependency.
+
 0.1.0
 =====
 
diff --git a/Setup.hs b/Setup.hs
deleted file mode 100644
--- a/Setup.hs
+++ /dev/null
@@ -1,2 +0,0 @@
-import Distribution.Simple
-main = defaultMain
diff --git a/o-clock.cabal b/o-clock.cabal
--- a/o-clock.cabal
+++ b/o-clock.cabal
@@ -1,5 +1,5 @@
 name:                o-clock
-version:             0.1.0
+version:             0.1.1
 synopsis:            Type-safe time library.
 description:         See README.md for details.
 homepage:            https://github.com/serokell/o-clock
@@ -15,7 +15,9 @@
                    , README.md
                    , README.lhs
 cabal-version:       >=2.0
-tested-with:         GHC == 8.4.1
+tested-with:         GHC == 8.0.2
+                   , GHC == 8.2.2
+                   , GHC == 8.4.1
 
 source-repository head
   type:     git
@@ -29,9 +31,8 @@
                          Time.Timestamp
                          Time.Units
   ghc-options:         -Wall
-  build-depends:       base         >= 4.10  && < 5
+  build-depends:       base         >= 4.9  && < 5
                      , ghc-prim     >= 0.5
-                     , transformers >= 0.5
   default-language:    Haskell2010
   default-extensions:  OverloadedStrings
                        RecordWildCards
@@ -42,7 +43,7 @@
 executable play-o-clock
   main-is:             Playground.hs
   build-depends:       o-clock
-                     , base     >= 4.10  && < 5
+                     , base     >= 4.9  && < 5
   hs-source-dirs:      examples
   default-language:    Haskell2010
   ghc-options:         -threaded -Wall
@@ -58,7 +59,7 @@
                        Test.Time.TypeSpec
                        Test.Time.Units
 
-  build-depends:       base            >= 4.10  && < 5
+  build-depends:       base            >= 4.9  && < 5
                      , o-clock
                      , hedgehog       ^>= 0.5.1
                      , tasty          ^>= 0.12
diff --git a/src/Time/Rational.hs b/src/Time/Rational.hs
--- a/src/Time/Rational.hs
+++ b/src/Time/Rational.hs
@@ -41,10 +41,17 @@
 import Data.Proxy (Proxy (..))
 import GHC.Natural (Natural)
 import GHC.Real (Ratio ((:%)))
+
 #if ( __GLASGOW_HASKELL__ >= 804 )
 import GHC.TypeNats (Div, Mod, type (<=?))
 #endif
+
+#if ( __GLASGOW_HASKELL__ >= 802 )
 import GHC.TypeNats (KnownNat, Nat, natVal)
+#else
+import GHC.TypeLits (KnownNat, Nat, natVal)
+#endif
+
 #if ( __GLASGOW_HASKELL__ >= 804 )
 import Unsafe.Coerce (unsafeCoerce)
 #endif
@@ -213,8 +220,11 @@
     ratVal :: RatioNat
 
 instance (KnownNat a, KnownNat b) => KnownRat (a :% b) where
+#if ( __GLASGOW_HASKELL__ >= 802 )
     ratVal = natVal (Proxy @a) :% natVal (Proxy @b)
-
+#else
+    ratVal = fromIntegral (natVal (Proxy @a)) :% fromIntegral (natVal (Proxy @b))
+#endif
 
 #if ( __GLASGOW_HASKELL__ >= 804 )
 newtype KnownRatDict (unit :: Rat) r = MkKnownRatDict (KnownRat unit => r)
diff --git a/src/Time/Timestamp.hs b/src/Time/Timestamp.hs
--- a/src/Time/Timestamp.hs
+++ b/src/Time/Timestamp.hs
@@ -59,7 +59,7 @@
          -> (Ordering, Time unit)
 timeDiff (Timestamp a) (Timestamp b) =
     let (order, r) = ratDiff a b
-    in (order, toUnit $ sec $ fromRational r)
+    in (order, toUnit $ sec r)
 
 {- | Returns the result of addition of 'Time' with 'Timestamp' elements.
 
@@ -134,7 +134,8 @@
 t1 +:+ t2 = toUnit t1 + t2
 {-# INLINE (+:+) #-}
 
--- | Substracts times of different units.
+-- | Substracts time amounts of different units. When the minuend is smaller
+-- than the subtrahend, this function will throw @Underflow :: ArithException@.
 --
 -- >>> minute 1 -:- sec 1
 -- 59s
@@ -147,7 +148,9 @@
 t1 -:- t2 = toUnit t1 - t2
 {-# INLINE (-:-) #-}
 
-{- | Similar to '-:-' but more safe and returns order in pair with resulting time difference.
+{- | Compute the difference between two amounts of time. The result is returned
+in two components: the ordering (which input is larger) and the numeric
+difference (how much larger). Unlike '-:-', does not throw @ArithException@.
 
 >>> sec 5 -%- sec 3
 (GT,2s)
@@ -164,12 +167,12 @@
 t1 -%- (Time t2Rat) =
     let (Time t1Rat) = toUnit @unitResult t1
         (order, rat) = ratDiff (toRational t1Rat) (toRational t2Rat)
-    in (order, Time $ fromRational rat)
+    in (order, Time rat)
 
-ratDiff :: Rational -> Rational -> (Ordering, Rational)
+ratDiff :: Rational -> Rational -> (Ordering, RatioNat)
 ratDiff r1 r2 =
     let order = compare r1 r2
-        diff  = case order of
+        diff  = fromRational $ case order of
                      LT -> r2 - r1
                      GT -> r1 - r2
                      EQ -> 0
diff --git a/src/Time/Units.hs b/src/Time/Units.hs
--- a/src/Time/Units.hs
+++ b/src/Time/Units.hs
@@ -62,7 +62,9 @@
 import Control.Monad (unless)
 import Control.Monad.IO.Class (MonadIO, liftIO)
 import Data.Char (isDigit, isLetter)
+import Data.Foldable (foldl')
 import Data.Proxy (Proxy (..))
+import Data.Semigroup (Semigroup (..))
 import GHC.Generics (Generic)
 import GHC.Natural (Natural)
 import GHC.Prim (coerce)
@@ -119,6 +121,23 @@
 -- | Time unit is represented as type level rational multiplier with kind 'Rat'.
 newtype Time (rat :: Rat) = Time { unTime :: RatioNat }
     deriving (Eq, Ord, Enum, Real, RealFrac, Generic)
+
+-- | Addition is associative binary operation for 'Semigroup' of 'Time'.
+instance Semigroup (Time (rat :: Rat)) where
+    (<>) = (+)
+    {-# INLINE (<>) #-}
+    sconcat = foldl' (<>) mempty
+    {-# INLINE sconcat #-}
+    stimes n (Time t) = Time (fromIntegral n * t)
+    {-# INLINE stimes #-}
+
+instance Monoid (Time (rat :: Rat)) where
+    mempty  = Time 0
+    {-# INLINE mempty #-}
+    mappend = (<>)
+    {-# INLINE mappend #-}
+    mconcat = foldl' (<>) mempty
+    {-# INLINE mconcat #-}
 
 -- | Type family for prettier 'show' of time units.
 type family UnitName (unit :: Rat) :: Symbol