tmparse: put in local timezone hack

Ctime is defined as printing a 3-character timezone
name. The timezone name is ambiguous. For example,
EST refers to both Australian and American eastern
time. On top of that, we don't want to make the
tzabbrev table exhaustive. So, we put in this hack:

Before we consult the well known table of timezones,
we check if the local time matches the timezone name.

On top of that, tm2sec

If you want unambiguous timezone parsing, use numeric
timezone offsets (Z, ZZ formats).

sys/man/2/tmdate

@@ -114,6 +114,9 @@ and the unpadded and padded complete value, respectively.
 .TP
 .B Z, ZZ, ZZZ
 The timezone in [+-]HHMM and [+-]HH:MM, and named form, respectively.
+If the named timezone matches the name of the local zone, then the
+local timezone will be used.
+Otherwise, we will attempt to use the named zones listed in RFC5322.
 .TP
 .B a, A
 Lower and uppercase 'am' and 'pm' specifiers, respectively.
@@ -267,6 +270,10 @@ print("%τ", &t); /*  Mon Nov 3 13:11:11 PST 2019 */
 
 .SH BUGS
 .PP
+Checking the timezone name against the local timezone is a
+dirty hack. The same date string may parse differently for
+people in different timezones.
+.PP
 There is no way to format specifier for subsecond precision.
 .PP
 The timezone information that we ship is out of date.

sys/src/libc/port/date.c

@@ -615,7 +615,7 @@ tmparse(Tm *tm, char *fmt, char *str, Tzone *tz, char **ep)
 	int depth, n, w, c0, zs, z0, z1, md, ampm, zoned, sloppy, tzo, ok;
 	vlong abs;
 	char *s, *p, *q;
-	Tzone *zparsed;
+	Tzone *zparsed, *local;
 	Tzabbrev *a;
 	Tzoffpair *m;
 
@@ -774,6 +774,32 @@ tmparse(Tm *tm, char *fmt, char *str, Tzone *tz, char **ep)
 			switch(w){
 			case -1:
 			case 3:
+				/*
+				 * Ugly Hack:
+				 * Ctime is defined as printing a 3-character timezone
+				 * name. The timezone name is ambiguous. For example,
+				 * EST refers to both Australian and American eastern
+				 * time. On top of that, we don't want to make the
+				 * tzabbrev table exhaustive. So, we put in this hack:
+				 *
+				 * Before we consult the well known table of timezones,
+				 * we check if the local time matches the timezone name.
+				 *
+				 * If you want unambiguous timezone parsing, use numeric
+				 * timezone offsets (Z, ZZ formats).
+				 */
+				if((local = tzload("local")) != nil){
+					if(cistrncmp(s, local->stname, strlen(local->stname)) == 0){
+						s += strlen(local->stname);
+						zparsed = local;
+						goto Zoneparsed;
+					}
+					if(cistrncmp(s, local->dlname, strlen(local->dlname)) == 0){
+						s += strlen(local->dlname);
+						zparsed = local;
+						goto Zoneparsed;
+					}
+				}
 				for(a = tzabbrev; a->abbr; a++){
 					n = strlen(a->abbr);
 					if(cistrncmp(s, a->abbr, n) == 0 && !isalpha(s[n]))