As i understand you have a start date were something should start to execute every forthnight,
there are several ways to do this, either you create a script that updates your crontab to
run another script ever forthnight, the initial script can be sceduled with at command.
Another approach is to not touch the crontab at all and only use at command, something
in line with script below called myscript.sh, and the you have a myaction.sh that does
what you want to be doing evert forthnight.
if [$# -eq 0]; then
at 8am Sep 23 myscript.sh 1
at 5:30am +14 days myscript.sh 1