From 6b0bf1ad387bc9b32cb76ff4519da09d82cd085d Mon Sep 17 00:00:00 2001
From: Martin Ridgers <martin.ridgers@epicgames.com>
Date: Wed, 20 Oct 2021 14:43:47 +0200
Subject: DateTime::Now() for POSIX platforms

---
 zencore/compactbinary.cpp | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

(limited to 'zencore/compactbinary.cpp')

diff --git a/zencore/compactbinary.cpp b/zencore/compactbinary.cpp
index ab0904b2b..041ad8861 100644
--- a/zencore/compactbinary.cpp
+++ b/zencore/compactbinary.cpp
@@ -18,6 +18,8 @@
 
 #if ZEN_PLATFORM_WINDOWS
 #	include <zencore/windows.h>
+#else
+#	include <time.h>
 #endif
 
 #if ZEN_WITH_TESTS
@@ -46,15 +48,31 @@ IsLeapYear(int Year)
 	return false;
 }
 
+static constexpr uint64_t
+GetPlatformToDateTimeBiasInSeconds()
+{
+#if ZEN_PLATFORM_WINDOWS
+	const uint64_t PlatformEpochYear	= 1601;
+#else
+	const uint64_t PlatformEpochYear	= 1970;
+#endif
+	const uint64_t DateTimeEpochYear	= 1;
+	return uint64_t(double(PlatformEpochYear - DateTimeEpochYear) * 365.2425) * 86400;
+}
+
 DateTime
 DateTime::Now()
 {
+	static const uint64_t EpochBias		  = GetPlatformToDateTimeBiasInSeconds();
+	static const uint64_t SecsTo100nsTicks = int64_t(10e9 / 100);
+
 #if ZEN_PLATFORM_WINDOWS
 	FILETIME SysTime;
 	GetSystemTimeAsFileTime(&SysTime);
-	return DateTime{504911232000000000ull + (uint64_t(SysTime.dwHighDateTime) << 32) | SysTime.dwLowDateTime};
+	return DateTime{(EpochBias * SecsTo100nsTicks) + (uint64_t(SysTime.dwHighDateTime) << 32) | SysTime.dwLowDateTime};
 #else
-#	error Needs implementation
+	int64_t SecondsSinceUnixEpoch = time(nullptr);
+	return DateTime{(EpochBias + SecondsSinceUnixEpoch) * SecsTo100nsTicks};
 #endif
 }
 
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