• ...so can't use DT interrupts to create a timer.
I guess this is a good place to start. Why not? You can use DT_INTs with every single Interrupt your PIC is capable, and all at the same time. You can have a Timer Interrupt and also multiple UART Interrupt Handlers, all with DT_INTs.

If you haven't already, read the book a few members assembled as a tutorial on DT_INTs:

http://dt.picbasic.co.uk/Site/Introduction