We present a new annotation scheme for normalizing time expressions, such as “three days ago”, to computer-readable forms, such as 2016-03-07. The annotation scheme addresses several weaknesses of the existing TimeML standard, allowing the representation of time expressions that align to more than one calendar unit (e.g., “the past three summers”), that are defined relative to events (e.g., “three weeks postoperative”), and that are unions or intersections of smaller time expressions (e.g., “Tuesdays and Thursdays”). It achieves this by modeling time expression interpretation as the semantic composition of temporal operators like UNION, NEXT, and AFTER. We have applied the annotation scheme to 34 documents so far, producing 1104 annotations, and achieving inter-annotator agreement of 0.821.