A string when not using an output range; void otherwise.
assert(Date.init.toISOExtString == "null"); assert(Date(2010, 7, 4).toISOExtString == "2010-07-04"); assert(Date(1998, 12, 25).toISOExtString == "1998-12-25"); assert(Date(0, 1, 5).toISOExtString == "0000-01-05"); assert(Date(-4, 1, 5).toISOExtString == "-0004-01-05");
Converts this Date to a string with the format YYYY-MM-DD. If writer is set, the resulting string will be written directly to it.