my solution

Extra1/Extra1.sln

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+
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio Version 17
+VisualStudioVersion = 17.14.36511.14 d17.14
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "Extra1", "Extra1.vcxproj", "{1D46C0B2-358B-4BBA-97D4-B8ADC2B70CBE}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|x64 = Debug|x64
+		Debug|x86 = Debug|x86
+		Release|x64 = Release|x64
+		Release|x86 = Release|x86
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{1D46C0B2-358B-4BBA-97D4-B8ADC2B70CBE}.Debug|x64.ActiveCfg = Debug|x64
+		{1D46C0B2-358B-4BBA-97D4-B8ADC2B70CBE}.Debug|x64.Build.0 = Debug|x64
+		{1D46C0B2-358B-4BBA-97D4-B8ADC2B70CBE}.Debug|x86.ActiveCfg = Debug|Win32
+		{1D46C0B2-358B-4BBA-97D4-B8ADC2B70CBE}.Debug|x86.Build.0 = Debug|Win32
+		{1D46C0B2-358B-4BBA-97D4-B8ADC2B70CBE}.Release|x64.ActiveCfg = Release|x64
+		{1D46C0B2-358B-4BBA-97D4-B8ADC2B70CBE}.Release|x64.Build.0 = Release|x64
+		{1D46C0B2-358B-4BBA-97D4-B8ADC2B70CBE}.Release|x86.ActiveCfg = Release|Win32
+		{1D46C0B2-358B-4BBA-97D4-B8ADC2B70CBE}.Release|x86.Build.0 = Release|Win32
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+	GlobalSection(ExtensibilityGlobals) = postSolution
+		SolutionGuid = {23589D41-ADFB-4FD9-BFE4-67AC42C56F62}
+	EndGlobalSection
+EndGlobal

Extra1/Extra1.vcxproj

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+<?xml version="1.0" encoding="utf-8"?>
+<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <ItemGroup Label="ProjectConfigurations">
+    <ProjectConfiguration Include="Debug|Win32">
+      <Configuration>Debug</Configuration>
+      <Platform>Win32</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="Release|Win32">
+      <Configuration>Release</Configuration>
+      <Platform>Win32</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="Debug|x64">
+      <Configuration>Debug</Configuration>
+      <Platform>x64</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="Release|x64">
+      <Configuration>Release</Configuration>
+      <Platform>x64</Platform>
+    </ProjectConfiguration>
+  </ItemGroup>
+  <PropertyGroup Label="Globals">
+    <VCProjectVersion>17.0</VCProjectVersion>
+    <Keyword>Win32Proj</Keyword>
+    <ProjectGuid>{1d46c0b2-358b-4bba-97d4-b8adc2b70cbe}</ProjectGuid>
+    <RootNamespace>Extra1</RootNamespace>
+    <WindowsTargetPlatformVersion>10.0</WindowsTargetPlatformVersion>
+  </PropertyGroup>
+  <Import Project="$(VCTargetsPath)\Microsoft.Cpp.Default.props" />
+  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'" Label="Configuration">
+    <ConfigurationType>Application</ConfigurationType>
+    <UseDebugLibraries>true</UseDebugLibraries>
+    <PlatformToolset>v143</PlatformToolset>
+    <CharacterSet>Unicode</CharacterSet>
+  </PropertyGroup>
+  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|Win32'" Label="Configuration">
+    <ConfigurationType>Application</ConfigurationType>
+    <UseDebugLibraries>false</UseDebugLibraries>
+    <PlatformToolset>v143</PlatformToolset>
+    <WholeProgramOptimization>true</WholeProgramOptimization>
+    <CharacterSet>Unicode</CharacterSet>
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+  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|x64'" Label="Configuration">
+    <ConfigurationType>Application</ConfigurationType>
+    <UseDebugLibraries>true</UseDebugLibraries>
+    <PlatformToolset>v143</PlatformToolset>
+    <CharacterSet>Unicode</CharacterSet>
+  </PropertyGroup>
+  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|x64'" Label="Configuration">
+    <ConfigurationType>Application</ConfigurationType>
+    <UseDebugLibraries>false</UseDebugLibraries>
+    <PlatformToolset>v143</PlatformToolset>
+    <WholeProgramOptimization>true</WholeProgramOptimization>
+    <CharacterSet>Unicode</CharacterSet>
+  </PropertyGroup>
+  <Import Project="$(VCTargetsPath)\Microsoft.Cpp.props" />
+  <ImportGroup Label="ExtensionSettings">
+  </ImportGroup>
+  <ImportGroup Label="Shared">
+  </ImportGroup>
+  <ImportGroup Label="PropertySheets" Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'">
+    <Import Project="$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props" Condition="exists('$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props')" Label="LocalAppDataPlatform" />
+  </ImportGroup>
+  <ImportGroup Label="PropertySheets" Condition="'$(Configuration)|$(Platform)'=='Release|Win32'">
+    <Import Project="$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props" Condition="exists('$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props')" Label="LocalAppDataPlatform" />
+  </ImportGroup>
+  <ImportGroup Label="PropertySheets" Condition="'$(Configuration)|$(Platform)'=='Debug|x64'">
+    <Import Project="$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props" Condition="exists('$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props')" Label="LocalAppDataPlatform" />
+  </ImportGroup>
+  <ImportGroup Label="PropertySheets" Condition="'$(Configuration)|$(Platform)'=='Release|x64'">
+    <Import Project="$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props" Condition="exists('$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props')" Label="LocalAppDataPlatform" />
+  </ImportGroup>
+  <PropertyGroup Label="UserMacros" />
+  <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'">
+    <ClCompile>
+      <WarningLevel>Level3</WarningLevel>
+      <SDLCheck>true</SDLCheck>
+      <PreprocessorDefinitions>WIN32;_DEBUG;_CONSOLE;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <ConformanceMode>true</ConformanceMode>
+    </ClCompile>
+    <Link>
+      <SubSystem>Console</SubSystem>
+      <GenerateDebugInformation>true</GenerateDebugInformation>
+    </Link>
+  </ItemDefinitionGroup>
+  <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Release|Win32'">
+    <ClCompile>
+      <WarningLevel>Level3</WarningLevel>
+      <FunctionLevelLinking>true</FunctionLevelLinking>
+      <IntrinsicFunctions>true</IntrinsicFunctions>
+      <SDLCheck>true</SDLCheck>
+      <PreprocessorDefinitions>WIN32;NDEBUG;_CONSOLE;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <ConformanceMode>true</ConformanceMode>
+    </ClCompile>
+    <Link>
+      <SubSystem>Console</SubSystem>
+      <GenerateDebugInformation>true</GenerateDebugInformation>
+    </Link>
+  </ItemDefinitionGroup>
+  <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Debug|x64'">
+    <ClCompile>
+      <WarningLevel>Level3</WarningLevel>
+      <SDLCheck>true</SDLCheck>
+      <PreprocessorDefinitions>_DEBUG;_CONSOLE;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <ConformanceMode>true</ConformanceMode>
+    </ClCompile>
+    <Link>
+      <SubSystem>Console</SubSystem>
+      <GenerateDebugInformation>true</GenerateDebugInformation>
+    </Link>
+  </ItemDefinitionGroup>
+  <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Release|x64'">
+    <ClCompile>
+      <WarningLevel>Level3</WarningLevel>
+      <FunctionLevelLinking>true</FunctionLevelLinking>
+      <IntrinsicFunctions>true</IntrinsicFunctions>
+      <SDLCheck>true</SDLCheck>
+      <PreprocessorDefinitions>NDEBUG;_CONSOLE;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <ConformanceMode>true</ConformanceMode>
+    </ClCompile>
+    <Link>
+      <SubSystem>Console</SubSystem>
+      <GenerateDebugInformation>true</GenerateDebugInformation>
+    </Link>
+  </ItemDefinitionGroup>
+  <ItemGroup>
+    <ClCompile Include="extra-task-1.cpp" />
+    <ClCompile Include="main.cpp" />
+  </ItemGroup>
+  <ItemGroup>
+    <ClInclude Include="extra-task-1.h" />
+  </ItemGroup>
+  <Import Project="$(VCTargetsPath)\Microsoft.Cpp.targets" />
+  <ImportGroup Label="ExtensionTargets">
+  </ImportGroup>
+</Project>

Extra1/Extra1.vcxproj.filters

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+<?xml version="1.0" encoding="utf-8"?>
+<Project ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <ItemGroup>
+    <Filter Include="Source Files">
+      <UniqueIdentifier>{4FC737F1-C7A5-4376-A066-2A32D752A2FF}</UniqueIdentifier>
+      <Extensions>cpp;c;cc;cxx;c++;cppm;ixx;def;odl;idl;hpj;bat;asm;asmx</Extensions>
+    </Filter>
+    <Filter Include="Header Files">
+      <UniqueIdentifier>{93995380-89BD-4b04-88EB-625FBE52EBFB}</UniqueIdentifier>
+      <Extensions>h;hh;hpp;hxx;h++;hm;inl;inc;ipp;xsd</Extensions>
+    </Filter>
+    <Filter Include="Resource Files">
+      <UniqueIdentifier>{67DA6AB6-F800-4c08-8B7A-83BB121AAD01}</UniqueIdentifier>
+      <Extensions>rc;ico;cur;bmp;dlg;rc2;rct;bin;rgs;gif;jpg;jpeg;jpe;resx;tiff;tif;png;wav;mfcribbon-ms</Extensions>
+    </Filter>
+  </ItemGroup>
+  <ItemGroup>
+    <ClCompile Include="extra-task-1.cpp">
+      <Filter>Source Files</Filter>
+    </ClCompile>
+    <ClCompile Include="main.cpp">
+      <Filter>Source Files</Filter>
+    </ClCompile>
+  </ItemGroup>
+  <ItemGroup>
+    <ClInclude Include="extra-task-1.h">
+      <Filter>Header Files</Filter>
+    </ClInclude>
+  </ItemGroup>
+</Project>

Extra1/extra-task-1.cpp

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+#include "extra-task-1.h"
+
+double seconds_difference(double time_1, double time_2)
+{
+	return time_2 - time_1;
+}
+
+double hours_difference(double time_1, double time_2)
+{
+	return seconds_difference(time_1, time_2) / 3600.0;
+}
+
+double to_float_hours(int hours, int minutes, int seconds)
+{
+	assert(0 <= minutes && minutes < 60 && 0 <= seconds && seconds < 60);
+    return hours + minutes / 60.0 + seconds / 3600.0;
+}
+
+double to_24_hour_clock(double hours)
+{
+	assert(hours >= 0);
+	return fmod(hours, 24.0);
+}
+
+int get_hours(int seconds)
+{
+    assert(seconds >= 0);
+    return to_24_hour_clock(seconds / 3600);
+}
+
+int get_minutes(int seconds)
+{
+    assert(seconds >= 0);
+    return (seconds % 3600) / 60;
+}
+
+int get_seconds(int seconds)
+{
+    assert(seconds >= 0);
+    return seconds % 60;
+}
+
+double time_to_utc(int utc_offset, double time)
+{
+	assert(0.0 <= time && time < 24.0);
+	double result = time - utc_offset;
+    result = fmod(result, 24.0);
+    if (result < 0)
+        result += 24.0;
+	return result;
+}
+
+double time_from_utc(int utc_offset, double time)
+{
+	assert(0.0 <= time && time < 24.0);
+    double result = time + utc_offset;
+    result = fmod(result, 24.0);
+    if (result < 0)
+        result += 24.0;
+    return result;
+}

Extra1/extra-task-1.h

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+#pragma once
+#include <cassert>
+#include <cmath>
+
+// Return the number of seconds that a time in seconds time_2 is later than a time in seconds time_1.
+double seconds_difference(double, double);
+
+// Return the number of hours that a time in seconds time_2 is later than a time in seconds time_1.
+double hours_difference(double, double);
+
+// Return the total number of hours in the specified number of hours, minutes, and seconds.
+double to_float_hours(int, int, int);
+
+// Convert a number of hours since midnight to a 24-hour clock format.
+double to_24_hour_clock(double);
+
+// Return the hours part of a time in seconds.
+int get_hours(int);
+
+// Return the minutes part of a time in seconds.
+int get_minutes(int);
+
+// Return the seconds part of a time in seconds.
+int get_seconds(int);
+
+// Return time at UTC+0, where utc_offset is the number of hours away from UTC+0.
+double time_to_utc(int, double);
+
+// Return UTC time in time zone utc_offset.
+double time_from_utc(int, double);

Extra1/main.cpp

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+#include "extra-task-1.h"
+#include <iostream>
+
+using namespace std;
+
+void assert_double_equal(double actual, double expected) {
+	assert(fabs(actual - expected) < DBL_EPSILON);
+}
+
+void main()
+{
+    assert_double_equal(seconds_difference(1800.0, 3600.0), 1800.0);
+    assert_double_equal(seconds_difference(3600.0, 1800.0), -1800.0);
+    assert_double_equal(seconds_difference(1800.0, 2160.0), 360.0);
+    assert_double_equal(seconds_difference(1800.0, 1800.0), 0.0);
+
+    assert_double_equal(hours_difference(1800.0, 3600.0), 0.5);
+    assert_double_equal(hours_difference(3600.0, 1800.0), -0.5);
+    assert_double_equal(hours_difference(1800.0, 2160.0), 0.1);
+    assert_double_equal(hours_difference(1800.0, 1800.0), 0.0);
+
+    assert_double_equal(to_float_hours(0, 15, 0), 0.25);
+    assert_double_equal(to_float_hours(2, 45, 9), 2.7525);
+    assert_double_equal(to_float_hours(1, 0, 36), 1.01);
+
+    assert_double_equal(to_24_hour_clock(24), 0);
+    assert_double_equal(to_24_hour_clock(48), 0);
+    assert_double_equal(to_24_hour_clock(25), 1);
+    assert_double_equal(to_24_hour_clock(4), 4);
+    assert_double_equal(to_24_hour_clock(28.5), 4.5);
+
+	assert_double_equal(get_hours(3800), 1);
+	assert_double_equal(get_minutes(3800), 3);
+	assert_double_equal(get_seconds(3800), 20);
+
+    assert_double_equal(time_to_utc(+0, 12.0), 12.0);
+    assert_double_equal(time_to_utc(+1, 12.0), 11.0);
+    assert_double_equal(time_to_utc(-1, 12.0), 13.0);
+    assert_double_equal(time_to_utc(-11, 18.0), 5.0);
+    assert_double_equal(time_to_utc(-1, 0.0), 1.0);
+    assert_double_equal(time_to_utc(-1, 23.0), 0.0);
+
+    assert_double_equal(time_from_utc(+0, 12.0), 12.0);
+    assert_double_equal(time_from_utc(+1, 12.0), 13.0);
+    assert_double_equal(time_from_utc(-1, 12.0), 11.0);
+    assert_double_equal(time_from_utc(+6, 6.0), 12.0);
+    assert_double_equal(time_from_utc(-7, 6.0), 23.0);
+    assert_double_equal(time_from_utc(-1, 0.0), 23.0);
+    assert_double_equal(time_from_utc(-1, 23.0), 22.0);
+    assert_double_equal(time_from_utc(+1, 23.0), 0.0);
+
+	cout << "All tests passed!" << endl;
+}