refactor: move away from gmtime

The next commit moves inherited code away from `gmtime` and removes it
altogether.

Author: kwvg

src/util/time.cpp

@@ -135,15 +135,12 @@ std::string FormatISO8601Date(int64_t nTime) {
     return strprintf("%04i-%02i-%02i", ts.tm_year + 1900, ts.tm_mon + 1, ts.tm_mday);
 }
 
-std::string FormatISO8601Time(int64_t nTime) {
-    struct tm ts;
-    time_t time_val = nTime;
-#ifdef HAVE_GMTIME_R
-    gmtime_r(&time_val, &ts);
-#else
-    gmtime_s(&ts, &time_val);
-#endif
-    return strprintf("%02i:%02i:%02iZ", ts.tm_hour, ts.tm_min, ts.tm_sec);
+std::string FormatISO8601Time(int64_t nTime)
+{
+    const std::chrono::sys_seconds secs{std::chrono::seconds{nTime}};
+    const auto days{std::chrono::floor<std::chrono::days>(secs)};
+    const std::chrono::hh_mm_ss hms{secs - days};
+    return strprintf("%02i:%02i:%02iZ", hms.hours().count(), hms.minutes().count(), hms.seconds().count());
 }
 
 int64_t ParseISO8601DateTime(const std::string& str)

src/wallet/wallet.cpp

@@ -3262,15 +3262,13 @@ bool CWallet::AutoBackupWallet(const fs::path& wallet_path, bilingual_str& error
     }
 
     // Create backup of the ...
-    struct tm ts;
-    time_t time_val = GetTime();
-#ifdef HAVE_GMTIME_R
-    gmtime_r(&time_val, &ts);
-#else
-    gmtime_s(&ts, &time_val);
-#endif
-    std::string dateTimeStr = strprintf(".%04i-%02i-%02i-%02i-%02i",
-            ts.tm_year + 1900, ts.tm_mon + 1, ts.tm_mday, ts.tm_hour, ts.tm_min);
+    std::string dateTimeStr = [&]() {
+        const std::chrono::sys_seconds secs{GetTime<std::chrono::seconds>()};
+        const auto days{std::chrono::floor<std::chrono::days>(secs)};
+        const std::chrono::year_month_day ymd{days};
+        const std::chrono::hh_mm_ss hms{secs - days};
+        return strprintf(".%04i-%02u-%02u-%02i-%02i", signed{ymd.year()}, unsigned{ymd.month()}, unsigned{ymd.day()}, hms.hours().count(), hms.minutes().count());
+    }();
 
     if (wallet_path.empty()) {
         // ... opened wallet