BPMN supports the design of intra-organization workflows and inter-organization collaborations. This rich notation includes elements to deal with models where time is central. However, the expressiveness of the BPMN time-related constructs hampers the definition of a formal semantics including them, and the provision of formal analysis means for timed process models. We propose here a first-order logic semantics for a subset of BPMN that includes its time-related constructs. With reference to related work, we support the specification of datetimes, durations, and cycles, using ISO-8601 formats as specified in the standard. Our approach is tool-supported by a model transformation into the Alloy formal language and its bounded counter-example generator. Our tool and model database are open source and freely available.