A string when not using an output range; void otherwise.
import core.time : msecs, hnsecs; import std.datetime.date : DateTime; assert(SysTime(DateTime(2010, 7, 4, 7, 6, 12)).toISOString() == "20100704T070612"); assert(SysTime(DateTime(1998, 12, 25, 2, 15, 0), msecs(24)).toISOString() == "19981225T021500.024"); assert(SysTime(DateTime(0, 1, 5, 23, 9, 59)).toISOString() == "00000105T230959"); assert(SysTime(DateTime(-4, 1, 5, 0, 0, 2), hnsecs(520_920)).toISOString() == "-00040105T000002.052092");
Converts this SysTime to a string with the format YYYYMMDDTHHMMSS.FFFFFFFTZ (where F is fractional seconds and TZ is time zone).
Note that the number of digits in the fractional seconds varies with the number of fractional seconds. It's a maximum of 7 (which would be hnsecs), but only has as many as are necessary to hold the correct value (so no trailing zeroes), and if there are no fractional seconds, then there is no decimal point.
If this SysTime's time zone is std.datetime.timezone.LocalTime, then TZ is empty. If its time zone is UTC, then it is "Z". Otherwise, it is the offset from UTC (e.g. +0100 or -0700). Note that the offset from UTC is not enough to uniquely identify the time zone.
Time zone offsets will be in the form +HHMM or -HHMM.
Warning: Previously, toISOString did the same as toISOExtString and generated +HH:MM or -HH:MM for the time zone when it was not std.datetime.timezone.LocalTime or std.datetime.timezone.UTC, which is not in conformance with ISO 8601 for the non-extended string format. This has now been fixed. However, for now, fromISOString will continue to accept the extended format for the time zone so that any code which has been writing out the result of toISOString to read in later will continue to work. The current behavior will be kept until July 2019 at which point, fromISOString will be fixed to be standards compliant.