On Macintosh and Windows, a time constant is enclosed by question marks (?...?). Times are specified in 24-hour format. Here are some examples of time constants:

?00:00:00? ` midnight
?09:30:00? ` 9:30 am
?13:01:59? ` 1 pm, 1 minute, and 59 seconds

On Macintosh, you can also use the Dagger symbol (Option + T on a US keyboard) instead of the question mark to denote a time constant.