Symbol time offset simulation
If you postulate that receiever's clock is perfect, then you want to make the transmitter send symbols every $T_s \pm \varepsilon$ seconds, where $T_s$ is the symbol period according to the receiver.
This is easily achieved by using a very high sampling rate in the transmitter. Let's assume $T_s=1$ and you need a deviation of $\pm 0.01$. This deviation corresponds to one sample, if the sampling interval is $0.01$. So, use a sampling rate of $1/.01 = 100$ samples per time unit, and have the transmitter send a new symbol every 99 samples (for a faster clock) or every 101 samples (for a slower clock). Then, have the receiver sample every 100 samples, and you're done.
By making the sampling rate fast enough, you can easily accomodate any delay you need. Using this method, it is also easy to simulate a time-varying clock drift in the transmitter and/or receiver.