BASH - at/cron/crontab online examples don't work. '

I am reading Advanced Bash Scripting by Mendell Cooper. In this eBook there is a Chapter 16 External Filters, Programs and Commands. Within this chapter the author mentions the commands "at" briefly. Since he mentions this without an example nor exercise, it may be that this is either not important, too difficult for a regular person to learn. or too easy.

I also looked at the following sites, and none of their examples worked:
1) http://www.thegeekstuff.com/2010/06/...mand-examples/
2) https://bugs.debian.org/cgi-bin/bugr...cgi?bug=460514

However, it is really a practical idea to fire off a certain command or script at regular fixed/repeating times. Thus I would dare to ask for some real functioning examples on how to use this command. Moreover, I would like to play a BigBen sound everyday at both noon and midnight (iff I am currently logged in).

Attempt 01:
$ date
四 5月 4 15:34:41 CST 2017
$ at 15:35:30 /usr/local/bin/ClockBell.sh

Result 01:
syntax error. Last token seen: :
Garbled time


FYI
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1
ClockBell.sh's contents: '

#!/bin/bash
#Purpose: To play a sound like Big Ben

mplayer /usr/share/sounds/custom/BigBenRings150_clip.mp3
wait

exit 0
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2
My OS:
trisquel-mini 7.0

Try these:

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I found how to use simple "at" commands

Success01: Fixed time, current date.
at 21:50 -f /usr/local/bin/ClockBell.sh
warning: commands will be executed using /bin/sh

Success02: Fixed time, future date.
at 21:55 May 4 2017 -f /usr/local/bin/ClockBell.sh

Success03:
at now + 1 min -f /usr/local/bin/ClockBell.sh

The -f option reads in the commands what are inside the given file, so that will only work with scripts.

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