diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,3 +1,16 @@
 # Changelog for pretty-relative-time
 
 ## Unreleased changes
+
+## [0.2.0.0] - 2020-02-12
+
+### Changed
+
+Fixed a bug like this:
+
+```
+ approximateDaysPerMonth :: Integral a => a
+-approximateDaysPerMonth = 30
++approximateDaysPerMonth = 31
+```
+
diff --git a/pretty-relative-time.cabal b/pretty-relative-time.cabal
--- a/pretty-relative-time.cabal
+++ b/pretty-relative-time.cabal
@@ -1,13 +1,13 @@
 cabal-version: 1.12
 
--- This file has been generated from package.yaml by hpack version 0.31.2.
+-- This file has been generated from package.yaml by hpack version 0.33.0.
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: adf64d540112ae1c5a3643234034c51572f5f251cba6a9c63ba8a48b8285b629
+-- hash: 3c3eba99b4fd54cfb105bc62e7caea77caf57513705abac5550c0b645bd43e96
 
 name:           pretty-relative-time
-version:        0.1.0.0
+version:        0.2.0.0
 synopsis:       Pretty relative time
 description:    Please see the README on Github at <https://github.com/NorfairKing/pretty-relative-time#readme>
 category:       Time
diff --git a/src/Text/Time/Pretty/Constants.hs b/src/Text/Time/Pretty/Constants.hs
--- a/src/Text/Time/Pretty/Constants.hs
+++ b/src/Text/Time/Pretty/Constants.hs
@@ -24,7 +24,7 @@
 daysPerWeek = 7
 
 approximateDaysPerMonth :: Integral a => a
-approximateDaysPerMonth = 30
+approximateDaysPerMonth = 31
 
 approximateDaysPerYear :: Integral a => a
 approximateDaysPerYear = 365
diff --git a/src/Text/Time/Pretty/TimeAgo.hs b/src/Text/Time/Pretty/TimeAgo.hs
--- a/src/Text/Time/Pretty/TimeAgo.hs
+++ b/src/Text/Time/Pretty/TimeAgo.hs
@@ -32,12 +32,22 @@
       [ genericValidate da
       , check
           (case daysAgoSign of
-             EQ -> and [daysAgoDays == 0, daysAgoWeeks == 0, daysAgoMonths == 0, daysAgoYears == 0]
-             _ -> any (> 0) [daysAgoDays, daysAgoWeeks, daysAgoMonths, daysAgoYears])
+             EQ ->
+               and
+                 [ daysAgoDays == 0
+                 , daysAgoWeeks == 0
+                 , daysAgoMonths == 0
+                 , daysAgoYears == 0
+                 ]
+             _ ->
+               any
+                 (> 0)
+                 [daysAgoDays, daysAgoWeeks, daysAgoMonths, daysAgoYears])
           "the sign makes sense"
       , check (daysAgoYears >= 0) "years are positive"
       , check
-          (daysAgoDays + daysPerWeek * daysAgoWeeks + approximateDaysPerMonth * daysAgoMonths <
+          (daysAgoDays + daysPerWeek * daysAgoWeeks +
+           approximateDaysPerMonth * daysAgoMonths <
            approximateDaysPerYear)
           "days, weeks and months do not sum to a year"
       , check (daysAgoMonths < 12) "months < 12"
@@ -59,7 +69,8 @@
     daysAgoYears = totalDays `div` approximateDaysPerYear
     daysLeftAfterYears = totalDays - daysAgoYears * approximateDaysPerYear
     daysAgoMonths = daysLeftAfterYears `div` approximateDaysPerMonth
-    daysLeftAfterMonths = daysLeftAfterYears - daysAgoMonths * approximateDaysPerMonth
+    daysLeftAfterMonths =
+      daysLeftAfterYears - daysAgoMonths * approximateDaysPerMonth
     daysAgoWeeks = daysLeftAfterMonths `div` daysPerWeek
     daysLeftAfterWeeks = daysLeftAfterMonths - daysAgoWeeks * daysPerWeek
     daysAgoDays = daysLeftAfterWeeks
@@ -70,7 +81,8 @@
      EQ -> const 0
      GT -> id
      LT -> negate) $
-  daysAgoDays + daysPerWeek * daysAgoWeeks + approximateDaysPerMonth * daysAgoMonths +
+  daysAgoDays + daysPerWeek * daysAgoWeeks +
+  approximateDaysPerMonth * daysAgoMonths +
   approximateDaysPerYear * daysAgoYears
 
 data TimeAgo =
@@ -123,17 +135,23 @@
 timeAgo dt = TimeAgo {..}
   where
     timeAgoSign = compare dt 0
-    timeAgoPicoSeconds = totalPicoSecondsAgo - picoSecondsPerSecond * totalSecondsAgo
+    timeAgoPicoSeconds =
+      totalPicoSecondsAgo - picoSecondsPerSecond * totalSecondsAgo
     timeAgoSeconds = totalSecondsAgo - secondsPerMinute * totalMinutesAgo
     timeAgoMinutes = totalMinutesAgo - minutesPerHour * totalHoursAgo
     timeAgoHours = totalHoursAgo - hoursPerDay * totalDaysAgo
     timeAgoDaysAgo = daysAgo totalDaysAgo
-    totalPicoSecondsAgo = floor $ absDt * fromIntegral (picoSecondsPerSecond :: Integer)
+    totalPicoSecondsAgo =
+      floor $ absDt * fromIntegral (picoSecondsPerSecond :: Integer)
     totalSecondsAgo = floor absDt :: Integer
     totalMinutesAgo = floor $ absDt / fromIntegral (secondsPerMinute :: Integer)
-    totalHoursAgo = floor $ absDt / fromIntegral (minutesPerHour * secondsPerMinute :: Integer)
+    totalHoursAgo =
+      floor $
+      absDt / fromIntegral (minutesPerHour * secondsPerMinute :: Integer)
     totalDaysAgo =
-      floor $ absDt / fromIntegral (hoursPerDay * minutesPerHour * secondsPerMinute :: Integer)
+      floor $
+      absDt /
+      fromIntegral (hoursPerDay * minutesPerHour * secondsPerMinute :: Integer)
     absDt = abs dt
 
 timeAgoToDiffTime :: TimeAgo -> NominalDiffTime
@@ -149,4 +167,5 @@
      (timeAgoSeconds +
       secondsPerMinute *
       (timeAgoMinutes +
-       minutesPerHour * (timeAgoHours + hoursPerDay * (daysAgoToDays timeAgoDaysAgo)))))
+       minutesPerHour *
+       (timeAgoHours + hoursPerDay * (daysAgoToDays timeAgoDaysAgo)))))