io: Redefine Clock.real to return timestamps relative to the POSIX/Unix epoch

`Clock.real` being defined to return timestamps relative to an
implementation-specific epochs means that there's currently no way for
the user to translate returned timestamps to actual calendar dates
without digging into implementation details of any particular `Io`
implementation. Redefining it to return timestamps relative to
1970-01-01 00:00:00+00:00 fixes this problem.

There are other ways to solve this, such as adding new vtable functions
for returning the implementation-specific epoch, but in terms of
complexity this redefinition is by far the simplest solution and only
amounts to a simple 96-bit integer addition's worth of overhead for
OSes like Windows that use non-POSIX/Unix epochs.

Author: castholm

lib/std/Io.zig

@@ -734,16 +734,19 @@ pub const Clock = enum {
     /// A settable system-wide clock that measures real (i.e. wall-clock)
     /// time. This clock is affected by discontinuous jumps in the system
     /// time (e.g., if the system administrator manually changes the
-    /// clock), and by frequency adjust‐ ments performed by NTP and similar
+    /// clock), and by frequency adjustments performed by NTP and similar
     /// applications.
     ///
     /// This clock normally counts the number of seconds since 1970-01-01
     /// 00:00:00 Coordinated Universal Time (UTC) except that it ignores
     /// leap seconds; near a leap second it is typically adjusted by NTP to
     /// stay roughly in sync with UTC.
     ///
-    /// The epoch is implementation-defined. For example NTFS/Windows uses
-    /// 1601-01-01.
+    /// Timestamps returned by this implementations of this clock are
+    /// relative to the POSIX/Unix epoch 1970-01-01 00:00:00+00:00
+    /// (colloquially referred to as "Unix time"). If the underlying OS
+    /// uses a different epoch for timestamp values (e.g., Windows, which
+    /// uses 1601-01-01) they are translated accordingly.
     real,
     /// A nonsettable system-wide clock that represents time since some
     /// unspecified point in the past.