Could you please share the screen-shot of your configuration? I could see only below actions available in SLA rule. I got little confusion 'i set Calculate service levels to "set to the value of a property"' this statement?
Action set available:
Q2. Why I can't use a date/time property in data page?
A. As per my knowledge, you can use Date/Time property in the data page. It's same as page property in Pega, just it acts as a cache memory to be shared across application/node/requestor as per configuration. You can add a date/time property in the class of the Data Page's object type and refer the same.
Q3. How to use Data Pages in SLA rules?
A. You can configure "Run Activity" action set and in the activity, you can call Data Page content as per your requirement.
Hope it answers your question, kindly notify by marking this post as answered/helpful for larger Pega audience.
There isn't a pages and classes tab, so that property needs to be in the same context as the SLA itself. Not only can you not use a data page, you can't use any other top level page. The solution is to have a property in that page context with the same value as your data page. Perhaps a reference property? I think what you are asking for (direct reference of a property on a data page) is an enhancement to the existing product.