Append bash-specific instructions to gigasecond

exercises/practice/gigasecond/.docs/instructions.append.md

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+# placeholder
+
+## How to solve with bash
+
+`bash` does not have any builtin ways to **parse** datetime strings.
+This exercise requires you to call out to an external utility.
+
+* The GNU `date` command's `-d` option can parse [a wide variety of formats][gnu-date].
+* For Mac users, see the [BSD `date` command][bsd-date] and its `-f` option.
+* Perl is widely available, and its [`Time::Piece` module][time-piece] can be used to parse timestamps.
+
+bash can **format** dates without any external tools.
+Given an integer time value (seconds since the epoch), [the `printf` `%()T` formatter][bash-printf] can apply "strftime"-style formatting.
+
+```bash
+epoch=1234567890
+
+printf '%(%Y-%m-%d %H:%M:%S)T\n' "$epoch"
+# ==> 2009-02-13 18:31:30
+
+# bash uses the `TZ` environment variable for the timezone.
+
+TZ=Asia/Kolkata printf '%(%Y-%m-%d %H:%M:%S)T\n' "$epoch"
+# ==> 2009-02-14 05:01:30
+```
+
+[gnu-date]: https://www.gnu.org/software/coreutils/manual/html_node/Date-input-formats.html#Date-input-formats
+[bsd-date]: https://manpage.me/index.cgi?apropos=0&q=date&sektion=0&manpath=FreeBSD+12-CURRENT+and+Ports&arch=default&format=html
+[time-piece]: https://perldoc.pl/Time::Piece
+[bash-printf]: https://www.gnu.org/software/bash/manual/bash.html#index-printf