I am currently applying for a postdoctoral research grant and one of the requirements is a schedule of research work. In experimental fields such as biology I can imagine how such schedule may be structured, say from this to that date conduct this or that experiment. In mathematics where no hypotheses are tested in the same sense as in experimental fields, no data are collected (in general) and detours or changes in approach are not uncommon, how can such a schedule be outlined?
I have never served on a grant committee, but I have experience as an applicant. Even though you obviously do not have any experiments or data collection planned, you surely have some idea of what you want to do, and how you want to approach that. And you know how many years you have available. By estimating the relative difficulties in each step, you should be able to come up with a rough idea of a timeline. What I believe they want to see is that you are able to give a realistic estimate of how difficult your proposed research is and how much time it will cost. You also might have different somewhat indepdendent projects in mind. Are you going to do the sequentially, or in parallel? Maybe you plan a research visit to another university. Answering such questions gives you a rough schedule that you can include in your proposal. I don't think it has to be very long or detailed.
Clearly during the course of the research it might be necessary to change the plan, because things did not work out as anticipated. But that is no different in other disciplines I am sure, and the reviewers are surely aware of that.
In short, the main goal in my opinion is that you should be able to show that you have a realistic assesment of how your proposed research can be completed in the alloted timeframe.