Change the behaviour of `-v' so that, e.g., stepping a month back

on March 31 won't take you to March 2 or 3 (now the result will
be the last day of February.)

In general, now stepping by months from the last days of the current
month A will take you to the very last day of the target month B if
B is shorter than A.

The previous version would just step to March 31 and rely on mktime(3)
to correct the date.  Despite its simplicity, such way was counter-intuitive
to users and caused pain to shell script writers.

Noticed by:	Igor Timkin <ivt at gamma dot ru>
Approved by:	brian
MFC after:	2 weeks

1 files changed, 7 insertions, 0 deletions

diff --git a/bin/date/vary.c b/bin/date/vary.c
index a314e48..5f01231 100644
--- a/bin/date/vary.c
+++ b/bin/date/vary.c
@@ -148,6 +148,8 @@ adjyear(struct tm *t, char type, int val, int mk)
 static int
 adjmon(struct tm *t, char type, int val, int istext, int mk)
 {
+  int lmdays;
+
   if (val < 0)
     return 0;
 
@@ -195,6 +197,11 @@ adjmon(struct tm *t, char type, int val, int istext, int mk)
       t->tm_mon = --val;
   }
 
+  /* e.g., -v-1m on March, 31 is the last day of February in common sense */
+  lmdays = daysinmonth(t);
+  if (t->tm_mday > lmdays)
+    t->tm_mday = lmdays;
+
   return !mk || domktime(t, type) != -1;
 }