If you feel that:
- It is to earlier: do it now. Usually we postpone some non-convenient but very useful activities. Preparing for university exam, etc.
- It is to be done now: wait. Usually we feel to do something right now only if it gives us pleasure and harm only. Or we just don't have enough information about consequences. E. g. if you see shining but used auto, I would like to buy it now just because it is beatiuful. But I should buy it later, spending some time looking under the hood.
- It is to late: do it now. It is never late for anything to do.