1. A computer-implemented method, comprising: determining, by a computing device, historical activities for a user; generating a decision tree for the user's historical activities, wherein each leaf node of the decision tree is associated with an activity-prediction model that computes a probability for a corresponding activity; selecting a path of the decision tree from a root node to a leaf node of the decision tree based on a target time, wherein the selected path traverses two or more non-leaf nodes that are each associated with a temporal decision model that compares the target time against a temporal classifier; and determining a probability for a user activity based on an activity-prediction model of the selected path.