Non-coherent energy-detection receivers are an attractive choice for IEEE 802.15.4a networks. They can exploit the ranging capabilities and the multipath resistance of impulse-radio ultra-wide band (IR-UWB) at a low complexity. However, IEEE 802.15.4a receivers operate with interference created by uncontrolled piconets and an uncoordinated medium access control layer. The performance of an energy-detection IR-UWB receiver is greatly degraded in such scenarios, for both timing acquisition and decoding. In this paper, we focus on timing acquisition: we present PICNIC, a robust and low-complexity algorithm that allows for reliable timing acquisition with an IR-UWB energy-detection receiver in the presence of multi-user interference (MUI), even in near-far scenarios. At the cost of a negligible performance reduction in single-user scenarios, PICNIC outperforms classic timing acquisition algorithms by up to two orders of magnitude if MUI is present. Furthermore, PICNIC exhibits a near perfect capture property: if several transmitters compete for timing acquisition at the receiver, one signal will be acquired with practically no false detection.