Creates a date-time value, with a date and time of day on that date.
The date part of the format is quite flexible, the main issue is avoid ambiguous dates. Dates like 4/8/2000 could be interpreted as 4th August or 8th April. This issue can be avoided in several ways: you can use the ISO format (e.g. datetime{2010-04-05 12:34} is 5th April), or you can use a month name, e.g. date{5 April 2010 4:56}
For the time part, you can use the 24-hour clock or AM/PM. Fractional seconds are allowed.