date time helper

diff --git a/datetime.sh b/datetime.sh
new file mode 100755 (executable)
index 0000000..2b4bba9
--- /dev/null
+++ b/datetime.sh
@@ -0,0 +1,101 @@
+#!/bin/sh
+
+[ "$include_datetime" ] && return 0
+include_datetime="$0"
+
+# Copyright 2023 - 2024 Paul Hänsch
+# 
+# Permission to use, copy, modify, and/or distribute this software for any
+# purpose with or without fee is hereby granted, provided that the above
+# copyright notice and this permission notice appear in all copies.
+# 
+# THE SOFTWARE IS PROVIDED “AS IS” AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+# WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+# MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+# SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+# WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+# ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR
+# IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+
+isdate(){
+  local date="$1" y m d
+
+  if   printf %s "$date" \
+    | grep -xEq '[0-9]{4}-((01|03|05|07|08|10|12)-(0[1-9]|[12][0-9]|3[01])|(04|06|09|11)-(0[1-9]|[12][0-9]|30)|02-(0[1-9]|[12][0-9]))'
+  then  # y-m-d (ISO Date)
+    y="${date%%-*}" d="${date##*-}" m="${date%-*}" m="${m#*-}"
+  elif printf %s "$date" \
+    | grep -xEq '((0?1|0?3|0?5|0?7|0?8|10|12)/(0?[1-9]|[12][0-9]|3[01])|(0?4|0?6|0?9|11)/(0?[1-9]|[12][0-9]|30)|0?2-(0[1-9]|[12][0-9]))/([0-9]{2}|[0-9]{4})'
+  then  # m/d/y (US Date)
+    y="${date##*/}" m="${date%%/*}" d="${date%/*}" d="${d#*/}"
+  elif printf %s "$date" \
+    | grep -xEq '((0?[1-9]|[12][0-9]|3[01])[\./](0?1|0?3|0?5|0?7|0?8|10|12)|(0?[1-9]|[12][0-9]|30)[\./](0?4|0?6|0?9|11)|(0[1-9]|[12][0-9])[\./]0?2)[\./]([0-9]{2}|[0-9]{4})'
+  then  # d/m/y or d.m.y (European Date / German Date)
+    y="${date##*.}" d="${date%%.*}" m="${date%.*}" m="${m#*.}"
+  else
+    return 1
+  fi
+  [ $y -lt 100 -a $y -gt 50 ] && y=$((y + 1900))
+  [ $y -lt 100 -a $y -le 50 ] && y=$((y + 2000))
+  date="$(printf "%04i-%02i-%02i" $y ${m#0} ${d#0})"
+
+  # leap year
+  if [ "$m" -eq 2 -a "$d" -eq 29 ]; then
+    if   [ "$((y % 400))" -eq 0 ]; then
+      :
+    elif [ "$((y % 100))" -eq 0 ]; then
+      return 1
+    elif [ "$((y % 4))" -eq 0 ]; then
+      :
+    else
+      return 1
+    fi
+  fi
+
+  printf '%04i-%02i-%02i\n' "$y" "${m#0}" "${d#0}"
+  return 0
+}
+
+istime(){
+  time="$1" h= m=
+
+  if   printf %s "$time" | grep -xEq '(0?[1-9]|1[012])(:[0-5][0-9])? ?(am|AM)\.?'; then
+    time="${time%?[aA][mM]}" h="${time%:*}" h="$(h % 12)"
+    [ "$h" != "$time" ] && m="${time#*:}" || m=0
+  elif printf %s "$time" | grep -xEq '(0?[1-9]|1[012])(:[0-5][0-9])? ?(pm|PM)\.?'; then
+    time="${time%?[aA][mM]}" h="${time%:*}" h="$(h % 12 + 12)"
+    [ "$h" != "$time" ] && m="${time#*:}" || m=0
+  elif printf %s "$time" | grep -xEq '(0?[0-9]|1[0-9]|2[0-3]):[0-5][0-9]'; then
+    time="${time%?[aA][mM]}" h="${time%:*}" m="${time#*:}"
+  else
+    return 1
+  fi
+
+  printf '%02i:%02i\n' "${h#0}" "${m#0}"
+  return 0
+}
+
+numdays(){
+  # return number of days in a month (i.e. 28, 29, 30, or 31)
+  local y="$1" m="${2#0}"
+
+  case $m in
+    1|3|5|7|10|12)
+      printf 31\\n
+      ;;
+    4|6|8|9|11)
+      printf 30\\n
+      ;;
+    2) if  [ "$((y % 400))" -eq 0 ]; then
+        printf 29\\n
+      elif [ "$((y % 100))" -eq 0 ]; then
+        printf 28\\n
+      elif [ "$((y % 4))" -eq 0 ]; then
+        printf 29\\n
+      else
+        printf 28\\n
+      fi
+      ;;
+    *) return 1;;
+  esac
+}