We used to manage ok in the Britishly Awesome 146. No winds in the box, no time markers, no VNAV, no seconds or decimal minutes on the FMS, no help. It was simple enough provided you understood that the FMS time did not round up at all, if your fix time was 1205 and the box said 1205 then you'd be overhead sometime between 1205:00 and 1205:59, i.e., you were going to be late. If the box said 1204 then you would be there between 1204:00 and 1204:59, half a chance you'd be there within the 30 seconds early buffer. A quick bit of maths to work out an average ground speed and then adjust indicated to achieve it. Unfortunately our only answer to "what's your planned indicated on descent?" from ATC was "haven't got a clue, just suck it and see!" I would err towards being early rather than late because it was easy for the bird to lose more time if asked but very difficult to make up any time.