Variables in linux Systemd Timer

One of my users wants to set up a timer that can take values as variables from a file.

Example.
1. File where variable value 'testvar="22-4-7"' is set - /home/user/timertest.conf
2. in the testing.timer, user wants to load $testvar value.

Location of the file with variable values can be anywhere, ive just given this as an example.

Simple example below.

Thank you

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$ nano /home/user/timertest.conf
~~~~~~~~~~~~~~
# content of /home/user/timertest.conf
testvar="22-4-7"
~~~~~~~~~~~~~

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$ /etc/systemd/system/testing.timer
~~~~~~~~~~~~~~~~~~~~~
# content of /etc/systemd/system/testing.timer
[Unit]
Description=testing

# load the content of /home/user/timertest.conf & use its value in OnCalender=${testvar}

[Timer]
OnCalendar=${testvar}
Unit=testing.service

[Install]
WantedBy=timers.target
~~~~~~~~~~~~~~~~~~~~~

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