Provide method for returning certificate expiration timestamp.

webrtc/base/timeutils.cc

Index: webrtc/base/timeutils.cc
diff --git a/webrtc/base/timeutils.cc b/webrtc/base/timeutils.cc
index fac5b66c7e005756d3c405db60752201ff27602f..16fbbe2c6f1c54b6bb8fb5c8512c1ace793c6494 100644
--- a/webrtc/base/timeutils.cc
+++ b/webrtc/base/timeutils.cc
@@ -204,4 +204,34 @@ int64_t TimestampWrapAroundHandler::Unwrap(uint32_t ts) {
   return unwrapped_ts;
 }
 
+int64_t TmToSeconds(const std::tm& tm) {
+  static short int cumul_days[12] = {0,   31,  59,  90,  120, 151,
+                                     181, 212, 243, 273, 304, 334};
+
+  int year = tm.tm_year + 1900;
+  int month = tm.tm_mon;
+  int day = tm.tm_mday - 1;  // Make 0-based like the rest.
+  int hour = tm.tm_hour;
+  int min = tm.tm_min;
+  int sec = tm.tm_sec;
+
+  day += cumul_days[month];

[nisse-webrtc, 2015/11/27 12:47:12]

I think you get an out-of-bounds read for invalid month in the input.

this functions needs to either handle out of range inputs properly, like month
15 or -15 spilling over to a different year, and similarly for day of month. Or
return -1 for all out-of-range inputs (which has the side-effect of also having
the only current caller of this function return -1 for invalid inputs. Special
care is needed for validating February 29, of course...). 

[torbjorng (webrtc), 2015/11/30 15:23:31]

Sure, this was intended as a low-level internal function for valid input...  But
validation never hurts (in not time-critical code) so I added it.

+
+  // Add number of leap days between 1970 and the expiration year, inclusive.
+  day += ((year / 4 - 1970 / 4) - (year / 100 - 1970 / 100) +
+          (year / 400 - 1970 / 400));
+
+  // If expiration time is in a leap year, number of days depends on exact date.
+  if (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)) {
+    if (month <= 2 - 1) {  // February or earlier (|month| is zero based).
+      day -= 1;

[nisse-webrtc, 2015/11/27 12:47:12]

This is a bit subtle... +1 for a leap year would be a bit more expected, I think
it's correct because if the current year is a leap year, that year's leap day is
already included in the leap day count a few lines up.

[torbjorng (webrtc), 2015/11/30 15:23:31]

Tried to clarify.

+    }
+  }
+
+  // Combine all variables into seconds from 1970-01-01 00:00 (except |month|
+  // which was accumulated into |day| above).
+  return (((static_cast<int64_t>
+            (year - 1970) * 365 + day) * 24 + hour) * 60 + min) * 60 + sec;
+}
+
 } // namespace rtc