Often a user knows only approximately when an event happened, when an interval began and ended, or even the duration of a span. For instance, she may know that an event happened ``between 2 PM and 4 PM,'' ``on Friday,'' ``sometime last week,'' or ``around the middle of the month.'' She may know that a airplane left ``on Friday'' and arrived ``on Saturday.'' Or perhaps, she has information that suggests that a graduate student takes ``four to fifteen'' years to write a dissertation. These are examples of time indeterminacy.