This position paper investigates some of the problems in modelling speech timing for the design of speech databases and corpus analysis tools for phonetics and speech technology. First we examine a selection of phonetic approaches to speech timing analysis, the so-called 'rhythm metrics', and focus on explaining (1) inconsistencies (varying results for the same language) and (2) the failure to model rhythmic alternation. To overcome these problems we present a new perspective on the phonetic identification of rhythm patterns as a special case of duration modelling, including the additional criterion of alternation. We describe the Rhythm Parser, a tool for identifying hierarchical alternating patterns, and discuss results from applying it.