Compute the year-ahead sqv policy date with a calendar

On 29 February the year-ahead date was 29 February of a year without one, which sqv rejects as a malformed timestamp, so every repository got a "Policy will reject signature within a year" warning that day. Fall back to the last day of February instead.

Closes: #1150291

methods/sqv.cc

@@ -3,6 +3,7 @@
 #include "aptmethod.h"
 #include <apt-pkg/gpgv.h>
 #include <apt-pkg/strutl.h>
+#include <chrono>
 #include <iterator>
 #include <optional>
 #include <ostream>
@@ -228,8 +229,13 @@ bool SQVMethod::VerifyGetSigners(const char *file, const char *outfile,
       _error->PushToStack();
       auto time = std::time(nullptr);
       auto tm = std::localtime(&time);
+      using namespace std::chrono;
+      auto yearAhead = year_month_day{year{tm->tm_year + 1900}, month(tm->tm_mon + 1), day(tm->tm_mday)} + years{1};
+      // 29 February has no counterpart in the next year
+      if (not yearAhead.ok())
+	 yearAhead = year_month_day_last{yearAhead.year(), month_day_last{yearAhead.month()}};
       std::string yearAheadDate;
-      strprintf(yearAheadDate, "%d-%d-%d", tm->tm_year + 1900 + 1, tm->tm_mon + 1, tm->tm_mday);
+      strprintf(yearAheadDate, "%d-%u-%u", int(yearAhead.year()), unsigned(yearAhead.month()), unsigned(yearAhead.day()));
 
       args.push_back("--policy-as-of");
       args.push_back(std::move(yearAheadDate));