Add Clocks::tz_offset.

GitOrigin-RevId: b1dcb9a8533f408961778eac7c7fa5db1bd2b920

tdutils/td/utils/port/Clocks.cpp

@@ -7,6 +7,7 @@
 #include "td/utils/port/Clocks.h"
 
 #include <chrono>
+#include <ctime>
 
 namespace td {
 
@@ -21,4 +22,40 @@ double Clocks::system() {
   return static_cast<double>(std::chrono::duration_cast<std::chrono::nanoseconds>(duration).count()) * 1e-9;
 }
 
+int Clocks::tz_offset() {
+  // not thread-safe on POSIX, so calculate the offset only once
+  static int offset = [] {
+    auto now = std::time(nullptr);
+
+    auto time_ptr = std::localtime(&now);
+    if (time_ptr == nullptr) {
+      return 0;
+    }
+    auto local_time = *time_ptr;
+
+    time_ptr = std::gmtime(&now);
+    if (time_ptr == nullptr) {
+      return 0;
+    }
+    auto utc_time = *time_ptr;
+
+    int minute_offset = local_time.tm_min - utc_time.tm_min;
+    int hour_offset = local_time.tm_hour - utc_time.tm_hour;
+    int day_offset = local_time.tm_mday - utc_time.tm_mday;
+    if (day_offset >= 20) {
+      day_offset = -1;
+    } else if (day_offset <= -20) {
+      day_offset = 1;
+    }
+    int sec_offset = day_offset * 86400 + hour_offset * 3600 + minute_offset * 60;
+    if (sec_offset >= 15 * 3600 || sec_offset <= -15 * 3600) {
+      return 0;
+    }
+    return sec_offset / 900 * 900;  // round to 900 just in case
+  }();
+  return offset;
+}
+
+static int init_tz_offset = Clocks::tz_offset();
+
 }  // namespace td

tdutils/td/utils/port/Clocks.h

@@ -12,6 +12,8 @@ struct Clocks {
   static double monotonic();
 
   static double system();
+
+  static int tz_offset();
 };
 
 }  // namespace td