From eab07866a3301bdd1c1def0ae891df1d4488c611 Mon Sep 17 00:00:00 2001
From: Jordan Woyak <jordan.woyak@gmail.com>
Date: Sun, 12 Jul 2020 12:36:58 -0500
Subject: [PATCH] Common/Timer: Fix integer underflow in Timer::GetDoubleTime
 for dates before ~2008.

---
 Source/Core/Common/Timer.cpp | 40 +++++++++---------------------------
 Source/Core/Common/Timer.h   |  2 +-
 2 files changed, 11 insertions(+), 31 deletions(-)

diff --git a/Source/Core/Common/Timer.cpp b/Source/Core/Common/Timer.cpp
index 794f2dfd49..3e953f24b4 100644
--- a/Source/Core/Common/Timer.cpp
+++ b/Source/Core/Common/Timer.cpp
@@ -4,7 +4,7 @@
 
 #include "Common/Timer.h"
 
-#include <ctime>
+#include <chrono>
 #include <string>
 
 #ifdef _WIN32
@@ -233,37 +233,17 @@ std::string Timer::GetTimeFormatted()
 // Returns a timestamp with decimals for precise time comparisons
 double Timer::GetDoubleTime()
 {
-#ifdef _WIN32
-  struct timeb tp;
-  (void)::ftime(&tp);
-#elif defined __APPLE__
-  struct timeval t;
-  (void)gettimeofday(&t, nullptr);
-#else
-  struct timespec t;
-  (void)clock_gettime(CLOCK_MONOTONIC, &t);
-#endif
-  // Get continuous timestamp
-  u64 TmpSeconds = Common::Timer::GetTimeSinceJan1970();
+  // FYI: std::chrono::system_clock epoch is not required to be 1970 until c++20.
+  // We will however assume time_t IS unix time.
+  using Clock = std::chrono::system_clock;
 
-  // Remove a few years. We only really want enough seconds to make
-  // sure that we are detecting actual actions, perhaps 60 seconds is
-  // enough really, but I leave a year of seconds anyway, in case the
-  // user's clock is incorrect or something like that.
-  TmpSeconds = TmpSeconds - DOUBLE_TIME_OFFSET;
+  // TODO: Use this on switch to c++20:
+  // const auto since_epoch = Clock::now().time_since_epoch();
+  const auto unix_epoch = Clock::from_time_t({});
+  const auto since_epoch = Clock::now() - unix_epoch;
 
-  // Make a smaller integer that fits in the double
-  u32 Seconds = (u32)TmpSeconds;
-#ifdef _WIN32
-  double ms = tp.millitm / 1000.0 / 1000.0;
-#elif defined __APPLE__
-  double ms = t.tv_usec / 1000000.0;
-#else
-  double ms = t.tv_nsec / 1000000000.0;
-#endif
-  double TmpTime = Seconds + ms;
-
-  return TmpTime;
+  const auto since_double_time_epoch = since_epoch - std::chrono::seconds(DOUBLE_TIME_OFFSET);
+  return std::chrono::duration_cast<std::chrono::duration<double>>(since_double_time_epoch).count();
 }
 
 // Formats a timestamp from GetDoubleTime() into a date and time string
diff --git a/Source/Core/Common/Timer.h b/Source/Core/Common/Timer.h
index aa82ebf6b0..021f94a736 100644
--- a/Source/Core/Common/Timer.h
+++ b/Source/Core/Common/Timer.h
@@ -41,7 +41,7 @@ public:
 
   // Arbitrarily chosen value (38 years) that is subtracted in GetDoubleTime()
   // to increase sub-second precision of the resulting double timestamp
-  static const int DOUBLE_TIME_OFFSET = (38 * 365 * 24 * 60 * 60);
+  static constexpr int DOUBLE_TIME_OFFSET = (38 * 365 * 24 * 60 * 60);
 
 private:
   u64 m_LastTime;