SLA schedule

I'm trying to deploy osticket for the very first time in our company. I did download the latest version and noticed that SLA plans do not have any scheduling. I need to configure it so in some defined days, some plans will have higher priority or even shorter time to expire.

I know there is such version like Ost+ but I already configured this installation, until it's necesarry, I would like to keep it original.

Can anyone help me?