diff --git a/lib/date/day-of-week.sh b/lib/date/day-of-week.sh
new file mode 100644
index 0000000..4af673f
--- /dev/null
+++ b/lib/date/day-of-week.sh
@@ -0,0 +1,31 @@
+# Prints the ISO day of week for a given date in the Gregorian calendar
+# 0 = Sunday, 1 = Monday, ..., 6 = Saturday
+#
+# This implementation works for all dates since the introduction of the
+# Gregorian calendar.
+#
+# Usage:
+#  - day-of-week year month day
+#  - day-of-week 1970 1 1
+#  - day-of-week 2020 2 29
+#  - day-of-week 2070 10 20
+
+: "${1:?missing year}" "${2:?missing month}" "${3:?missing day}"
+# remove leading zeroes from arguments to prevent them from being interpreted
+# as octal. The year is ignored because the Gregorian calendar was only
+# introduced in the 16th century.
+set -- "${1}" "${2#0}" "${3#0}"
+
+# RFC 3339, Appendix B
+
+set -- $(($2 >= 3 ? $1 : ($1 - 1))) $(($2 >= 3 ? ($2 - 2) : ($2 + 10))) $(($3))
+echo $((
+	(
+		(26 * $2 - 2) / 10
+		+ $3
+		+ ($1 % 100)
+		+ (($1 % 100) / 4)
+		+ ($1 / 400)
+		+ (5 * ($1 / 100))
+	) % 7
+))
diff --git a/spec/date/day-of-week.spec b/spec/date/day-of-week.spec
new file mode 100644
index 0000000..d3b379b
--- /dev/null
+++ b/spec/date/day-of-week.spec
@@ -0,0 +1,106 @@
+Describe 'date/day-of-week'
+  EnableSandbox
+
+  EnableLeakDetector
+
+  SetupCommandFromFile day-of-week lib/date/day-of-week.sh
+
+  # check if GNU date(1) is available
+  case $(@gdate --version 2>/dev/null)
+  in
+    (*'GNU coreutils'*)
+      __have_gdate=1 ;;
+    (*)
+      __have_gdate=0 ;;
+  esac
+  have_gdate() {
+    return $((! __have_gdate))
+  }
+  skip_gdate() {
+    return $((__have_gdate))
+  }
+
+  # check if this shell has $RANDOM
+  have_random_var() {
+    case ${RANDOM-} in ('') ! : ;; esac
+  }
+
+
+  Context  # static test values
+    Parameters
+      # year mon day    expected day-of-week
+        1970   1   1    4
+        1900   1   1    1
+        1900   2  28    3
+        1900   2  29    4  # NOTE: this is an invalid date
+        1900   3   1    4
+        2000   1   1    6
+        2000   2  28    1
+        2000   2  29    2
+        2000   3   1    3
+        2020   3   1    0
+        2024   2   1    4
+        2024   2  28    3
+        2024   2  29    4
+        2024   3   1    5
+    End
+
+    It "calculates the day of week for $(@printf '%04u-%02u-%02u' $1 $2 $3)"
+      When run command day-of-week $1 $2 $3
+
+      The status should equal 0
+      The stdout should equal "$4"
+      The stderr should equal ''
+    End
+  End
+
+  It 'supports parameters with leading 0s'
+    When run command day-of-week 1970 01 01
+
+    The status should be success
+    The output should equal 4
+    The error should equal ''
+  End
+
+  Context  # dynamic test values
+    Parameters:dynamic
+      n=10
+
+      if case ${RANDOM-} in ('') ! : ;; esac
+      then
+        while test $((n-=1)) -ge 0
+        do
+          Y=$(( 1600 + (RANDOM % 1000) ))
+          m=$(( 1 + (RANDOM % 12) ))
+          case ${m}
+          in
+            (2)
+              d=$(( 1 + (RANDOM % (28 + (0==(Y % 4)&&(0!=(Y % 100)||0==(Y % 400))))) )) ;;
+            (1|3|5|7|8|10|12)
+              d=$(( 1 + (RANDOM % 31) )) ;;
+            (*)
+              d=$(( 1 + (RANDOM % 30) )) ;;
+          esac
+            %data $Y $m $d
+        done
+      else
+        :  # skip: cannot generate random dates
+      fi
+    End
+
+    gdate_check_dow() {
+      test "${gdate_check_dow}" -eq $(($(TZ=GMT0 @gdate --date="$1" +%u) % 7))
+    }
+
+    It "calculates the day of week for $(@printf '%04u-%02u-%02u' $1 $2 $3) (dynamic)"
+      Skip if 'date(1) is not GNU date' skip_gdate
+
+      When run command day-of-week $1 $2 $3
+
+      The status should equal 0
+      # NOTE: must check using function to call @gdate only if it is available
+      The stdout should satisfy gdate_check_dow "$(@printf '%04u-%02u-%02u' $1 $2 $3)"
+      The stderr should equal ''
+    End
+  End
+End
diff --git a/spec/support/bin/@gdate b/spec/support/bin/@gdate
new file mode 100755
index 0000000..8af9be8
--- /dev/null
+++ b/spec/support/bin/@gdate
@@ -0,0 +1,7 @@
+#!/bin/sh -e
+. "$SHELLSPEC_SUPPORT_BIN"
+case ${OSTYPE}
+in
+	(*darwin*) invoke gdate "$@" ;;
+	(*) invoke date "$@" ;;
+esac