If a format string lacks a \fB%z\fR or \fB%Z\fR format group,
it is possible for the time to be ambiguous because it appears twice
in the same day, once without and once with Daylight Saving Time.
If this situation occurs, the first occurrence of the time is chosen.
(For this reason, it is wise to have the input string contain the
time zone when converting local times.  This caveat does not apply to
UTC times.)
.PP
If the interpretation of the groups yields an impossible time because
a field is out of range, enough of that field's unit will be added to
or subtracted from the time to bring it in range. Thus, if attempting to
scan or format day 0 of the month, one day will be subtracted from day
1 of the month, yielding the last day of the previous month.
.PP
If the interpretation of the groups yields an impossible time because
a Daylight Saving Time change skips over that time, or an ambiguous
time because a Daylight Saving Time change skips back so that the clock
observes the given time twice, and no time zone specifier (\fB%z\fR
or \fB%Z\fR) is present in the format, the time is interpreted as
if the clock had not changed. 
.SH "FORMAT GROUPS"
.PP
The following format groups are recognized by the \fBclock scan\fR and
\fBclock format\fR commands.
.TP
\fB%a\fR
On output, receives an abbreviation (\fIe.g.,\fR \fBMon\fR) for the day