Electromagnetic Interferences (EMI) are subject to very strict regulations by regulatory bodies. These regulations aim at limiting the amount of EMI electronic devices emit, and at preventing interference between electronic devices, and possible damages to the human body. With digital clock frequencies in excess of 100 MHz, radiated emissions become more of a problem. In modern integrated circuits, the greatest amount of electromagnetic radiation is created by the digital clock and signals derived from the clock. This is due to the nature of the clock signal, which is periodic with the highest frequency. Therefore, the energy concentrated at discrete frequencies (odd harmonics of this clock) is radiated off-chip.
This radiation is reduced by EMI reduction methods like (shielding, special coating, filtering components, etc.) or newer techniques like altering the spectral profile of the digital clock, which significantly reduces the electromagnetic radiation by making the clock becomes composed of a greater range of frequencies of lesser amplitude i.e. essentially the clock gets broadbanded and its energy gets spread over a larger bandwidth.
Even then, it may be prudent to keep digital clocks at least a foot away.