First of all, 2036 is just a GUI limitation. Time and date are managed by the Linux kernel so it will continue working after 2036 but you may have to use the date command to set it if the GUI hasn't been fixed in CSSU by then.

The real overflow will occur in 2038 as already mentioned. Rest assured, the problem is being worked on. I would hope that all N900 patches would have been upstreamed by then. The latest upstream kernels are already bootable on the N900 with many things working, no patches required (at least under Debian).