feat(user/calcGalTick):Add script to calculate Galactic Ticks

diff --git a/user/calcGalTick.sh b/user/calcGalTick.sh
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+++ b/user/calcGalTick.sh
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+#!/bin/bash
+# Desc: Prints a list of galactic tick dates
+# Usage: ./calcGalTick.sh [int start] [int end]
+# Input: arg1: int   start index
+#        arg2: int   end index
+# Example: ./calcGalTick.sh 0 250
+# Depends: bc 1.07.1; date (GNU coreutils) 8.32
+# Version: 0.0.1
+# Ref/Attrib: [1] Galactic Tick Day Calendar. (2021-10-20). https://github.com/GalacticTick/GalacticTick.github.io/blob/master/documents/press-3.pdf
+
+# Galactic Tick duration equation (seconds)
+## (Earth years per galactic turn) / (100ths of arcseconds per turn) * (solar days in Earth year)
+sGalTickEq="(225 * 10^6) / (1296000 * 100) * (365.25) * (24*3600)";
+#sGalTickEq="(225 * 10^6) / (1296000 * 100) * (365.24217) * (24*3600)";
+
+# Galactic tick duration (seconds)
+sGalTick="$(echo "$sGalTickEq" | bc -l)"; # 54788453.99999999999996457600
+#sGalTick="$(printf "%.0f" "$sGalTick")"; # round to int
+
+# Unix epoch of first tick (seconds; date Hans Lippershey patented the first telescope)
+ueFirstTick="$(date --date="1608-10-02" +%s)";
+
+yell() { echo "$0: $*" >&2; } # print script path and all args to stderr
+die() { yell "$*"; exit 111; } # same as yell() but non-zero exit status
+must() { "$@" || die "cannot $*"; } # runs args as command, reports args if command fails
+printGalTickDate() {
+    # Usage: printGalTickDate [int]
+    # Input:  arg1: nthGalTick  int  n'th observance since frist Galactic Tick
+    #         var: sGalTick:    int   Galactic tick duration
+    #         var: ueFirstTick: int  Unix epoch of first tick
+    # Output: str: ISO-8601 date string
+    # Example: printGalTickDate 239
+    #   2023-09-09
+    # Depends: bc 1.07.1; date (GNU coreutils) 8.32
+    # Version: 0.0.1
+    local nthGalTick output;
+
+    # Calc Unix epoch of n'th observance since first Galactic Tick
+    ## 0th tick was first tick
+    nthGalTick="$1";
+    ueTickEq="( $ueFirstTick + ( ($nthGalTick - 0) * $sGalTick ) )";
+    ueTick="$( echo "$ueTickEq" | bc -l )";
+
+    # Print calendar date
+    #output="$(date --date="@$ueTick" +%Y-%m-%dT%H:%M:%S%z)";
+    output="$(date --date="@$ueTick" +%Y-%m-%d)";
+
+    printf "%s\n" "$output";
+}; # print ISO-8601 calendar date of tick
+main() {
+    start="$1";
+    end="$2";
+    for ((tick="$start"; tick<="$end"; tick++)); do
+        printf "%03d: " "$tick"; # observance number
+        printGalTickDate "$tick"; # date
+    done;
+}; # main program
+
+main "$@";
+
+# Author: Steven Baltakatei Sandoval
+# License: GPLv3+