Improve easter egg for 20 years of grml.org

Relevant changes:

1) Don't convert dates via date(1), but provide epoch seconds right away
2) Also use zsh's ${EPOCHSECONDS} instead of forking to date(1)
3) Display easter egg message with einfo iff we are within the birthday range
4) Don't display easter egg only on 2023-09-16 and one month later,
   but also in all the days in between (sigh :))
5) Don't display anything at all when booting with noeasteregg boot option

Thanks: Christopher Bock and András Korn

autoconfig.functions

@@ -1994,17 +1994,24 @@ fi
 
 # {{{ Easteregg (for 20 years grml.org)
 config_easteregg() {
-  current_date=$(date +%Y-%m-%d)
-  birthday="2023-09-16"
-  one_month_later=$(date -d "${current_date} + 1 month" +%Y-%m-%d)
+  if checkbootparam 'noeasteregg'; then
+    return 0
+  fi
 
-  einfo "You found the birthday easter egg!" ; eend 0
+  local birthday current_date one_month_later
+  zmodload zsh/datetime
+
+  birthday='1694822400'          # := 2023-09-16 -> TZ=UTC date -d "2023-09-16" +%s
+  one_month_later='1697407200'   # := 2023-10-16 -> TC=UTC date -d "2023-09-16 + 1 month" +%s
+  current_date="${EPOCHSECONDS}" # simulate: current_date="$(date -d 2024-09-16 +%s)"
 
-  # nothing to be done if it's more than one month since $birthday
-  if ! [[ "${current_date}" == "${birthday}" || "${current_date}" == "${one_month_later}" ]]; then
+  # skip output before birthday or after one month has passend
+  if ! [[ "${current_date}" -ge "${birthday}" && "${current_date}" -lt "${one_month_later}" ]]; then
     return 0
   fi
 
+  einfo "You found the birthday easter egg!" ; eend 0
+
   if [[ -x /bin/toilet && -x /usr/games/lolcat ]] ; then
     visualize() { printf "%s\n" "$*" | toilet | /usr/games/lolcat ; }
   elif [[ -x /bin/toilet ]] ; then
@@ -2017,6 +2024,7 @@ config_easteregg() {
   visualize "20 years"
   visualize "grml.org"
 }
+
 # }}}
 
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