What does it mean that HDLs "explicitly include the notion of time"?
Programming languages usually don't have a notion of time.  They describe how things are sequenced, but usually they don't go further.  When they do, that's relegated to the library part of the language, and it is to handle time computation or to handle real time measurement and delay.

HDL are used for two purposes: synthesis and simulation.  For the first purpose, there is no more use of time than for a programming language.  For the second purpose, time is inherently part of what is to be simulated.  You need to know after which delay $Q$ will change after a change of $R$, and after which delay $\bar Q$ will do the same.  The time here is thus not the real time, but a simulated one, which has no easy relationship with the real time during which the simulation will occurs.

Your circuit will be described in a VHDL-like syntax:

Q &lt;= not (R or QBar) delay 1ns;
QBar &lt;= not (S or Q) delay 1ns;


You can also write an oscillator, with no stable state:

Clock &lt;= not Clock delay 5ns;


Time is thus present in the syntax of the language, and in its semantic as well (the language clearly states that $\bar Q$ will change 2ns of simulated time after $R$ does).