Is a Real-Time Clock (RTC) necessary for real-time systems?
The article you linked is just complete and utter nonsense. The "real time" in "real time clock" (as it's used to refer to the type of hardward device described in the article) and the "real time" in "real time systems" are completely different terms. The former means storing the current calendar time (usually some very poor approximation of it, as opposed to high-precision like the linked article claimed) and advancing it without external power, using a long-life button/coin type battery. The latter means responding to events with hard bounds on latency from the time of the event to the time of the response.

A few other bits from the article, to establish that it should be regarded as untrustworthy:


  Almost negligible. Of the order of 1 sec in 100 years


1 sec in 100 years is roughly 317 ppt (yes, that's parts per trillion). You can't get that kind of clock stability with any existing commercially-available clock technology. Even getting it to 1 second per year would require at least an OCXO which requires a high-power, always-on oven regulating the temperature. The idea you could get it with a device powered by long-life coin battery is laughable.


  real time systems like digital clock, attendance system, digital camera


None of these are what one would call real time systems.