diff --git a/src/Data/TimeDomain.hs b/src/Data/TimeDomain.hs
--- a/src/Data/TimeDomain.hs
+++ b/src/Data/TimeDomain.hs
@@ -17,15 +17,17 @@
   where
 
 -- time
-import Data.Time.Clock (UTCTime, diffUTCTime)
+import Data.Time.Clock (UTCTime, diffUTCTime, addUTCTime)
 
 {- |
 A time domain is an affine space representing a notion of time,
 such as real time, simulated time, steps, or a completely different notion.
 
-Expected law:
+Expected laws:
 
-@(t1 `diffTime` t3) `difference` (t1 `diffTime` t2) = t2 `diffTime` t3@
+* @(t1 `diffTime` t3) `difference` (t1 `diffTime` t2) = t2 `diffTime` t3@
+* @(t `addTime` dt) `diffTime` t = dt@
+* @(t `addTime` dt1) `addTime` dt2 = t `addTime` (dt1 `add` dt2)@
 -}
 class TimeDifference (Diff time) => TimeDomain time where
   -- | The type of differences or durations between two timestamps
@@ -35,51 +37,70 @@
 
   Mnemonic: 'diffTime' behaves like the '(-)' operator:
 
-  @'diffTime' earlier later = later `'diffTime'` earlier@ is the duration it takes from @earlier@ to @later.
+  @'diffTime' earlier later = later `'diffTime'` earlier@ is the duration it takes from @earlier@ to @later@.
   -}
   diffTime :: time -> time -> Diff time
 
+  {- | Add a time difference to a timestamp.
+  -}
+  addTime :: time -> Diff time -> time
+
 {- | A type of durations, or differences betweens time stamps.
 
-For the expected law, see 'TimeDomain'.
+Expected laws:
+
+* `add` is commutative and associative
+* @(dt1 `difference` dt2) `add` dt2 = dt1@
 -}
 class TimeDifference d where
   -- | Calculate the difference between two durations,
   --   compatibly with 'diffTime'.
   difference :: d -> d -> d
 
+  -- | Add two time differences.
+  add :: d -> d -> d
+
 -- | Differences between 'UTCTime's are measured in seconds.
 instance TimeDomain UTCTime where
   type Diff UTCTime = Double
   diffTime t1 t2 = realToFrac $ diffUTCTime t1 t2
+  addTime = flip $ addUTCTime . realToFrac
 
 instance TimeDifference Double where
   difference = (-)
+  add = (+)
 
 instance TimeDomain Double where
   type Diff Double = Double
   diffTime = (-)
+  addTime = (+)
 
 instance TimeDifference Float where
   difference = (-)
+  add = (+)
 
 instance TimeDomain Float where
   type Diff Float = Float
   diffTime = (-)
+  addTime = (+)
 
 instance TimeDifference Integer where
   difference = (-)
+  add = (+)
 
 instance TimeDomain Integer where
   type Diff Integer = Integer
   diffTime = (-)
+  addTime = (+)
 
 instance TimeDifference () where
   difference _ _ = ()
+  add _ _ = ()
 
 instance TimeDomain () where
   type Diff () = ()
   diffTime _ _ = ()
+  addTime _ _ = ()
 
 -- | Any 'Num' can be wrapped to form a 'TimeDomain'.
 newtype NumTimeDomain a = NumTimeDomain { fromNumTimeDomain :: a }
@@ -87,7 +108,9 @@
 
 instance Num a => TimeDifference (NumTimeDomain a) where
   difference = (-)
+  add = (+)
 
 instance Num a => TimeDomain (NumTimeDomain a) where
   type Diff (NumTimeDomain a) = NumTimeDomain a
   diffTime = (-)
+  addTime = (+)
diff --git a/time-domain.cabal b/time-domain.cabal
--- a/time-domain.cabal
+++ b/time-domain.cabal
@@ -1,6 +1,6 @@
 cabal-version:      2.4
 name:               time-domain
-version:            0.1.0.0
+version:            0.1.0.1
 license:            MIT
 license-file:       LICENSE
 author:             Manuel Bärenz
@@ -12,11 +12,18 @@
 extra-source-files: CHANGELOG.md
 category:           FRP
 homepage:           https://github.com/turion/time-domain/
+source-repository head
+  type:     git
+  location: git@github.com:turion/time-domain.git
+source-repository this
+  type:     git
+  location: git@github.com:turion/time-domain.git
+  tag:      v0.1.0.1
 
 library
   exposed-modules:  Data.TimeDomain
   build-depends:
-      base >= 4.13.0 && <= 4.17
+      base >= 4.13.0 && <= 4.18
     , time >= 1.9
   hs-source-dirs:   src
   default-language: Haskell2010