RESET does not touch the RTC, but it does reset the DM to 0 If you select not to reset then the PLC will not reset all the IOs and DM and you can continue execution of the program from where it last paused before new program is transferred.
If your PLC firmware is r47 and above (it should be if it was purchased in the last two years), you could also use the SETSYSTEM 5,k command to preserve the values of DM during a reset. Run it once and the DM from DM[1] to DM[k] will be skipped from the RESET routine.