'RTC' = 'real time clock', so it's the 'back-up' (CMOS) battery inside the laptop, which keeps things in memory even when it's switched off', which appears to have reached the end of its life.
http://www.laptopparts101.com/cmos-rtc-battery/
You need one of these
http://genuine-cmos-battery.co.uk/toshiba-satellite-l850_00q-cmos-battery.html
and either the confidence to fit it yourself or the services of a local computer technician. (It's a few minutes work. DON'T go to PC World, as they'll charge you £50, plus the cost of the part, for those few minutes. Find a local guy!)