The problem of executing sequential programs optimistically using the Time Warp algorithm is considered. It is shown how to do this, by first mapping the sequential execution to a control tree and then assigning timestamps to each node in the tree. For such timestamps to be effective they must be finite, this implies that they must be periodically rescaled to allow old timestamps to be reused. A number of timestamp representations are described and compared on the basis of: their complexity; the frequency and cost of rescaling; and the cost of performing basic operations, including comparison and creation of new timestamps.