WorkTimeAndBreaks.cs

var startTime = new DateTime(2022, 4, 11, 8, 0, 0);
var endTime = new DateTime(2022, 4, 11, 17, 0, 0);

var breaks = new List<(DateTime Start, DateTime End)>
{
    (new DateTime(2022, 4, 11, 12, 0, 0), new DateTime(2022, 4, 11, 13, 0, 0)),
    (new DateTime(2022, 4, 11, 15, 0, 0), new DateTime(2022, 4, 11, 15, 30, 0))
};

// Validate breaks, merge them if needed


Console.WriteLine($"Worktime: {startTime} - {endTime}");

foreach (var item in breaks)
{
    Console.WriteLine($"Break: {item.Start} - {item.End}");
}

var workingTime = new List<(DateTime Start, DateTime End)>();

var firstBreakStart = GetFirstBreak();

workingTime.Add(new(startTime, firstBreakStart == DateTime.MinValue ? endTime : firstBreakStart));

// no breaks
if (firstBreakStart == DateTime.MinValue)
{
    ShowResult();
    return;
};

for (var i = 0; i < breaks.Count - 1; i++)
{
    workingTime.Add(new(breaks[i].End, breaks[i + 1].Start));
}

var lastBreakEnd = GetLastBreak();

if (lastBreakEnd != DateTime.MinValue) workingTime.Add(new(lastBreakEnd, endTime));

ShowResult();

DateTime GetFirstBreak() => breaks.OrderBy(s => s.Start).FirstOrDefault().Start;
DateTime GetLastBreak() => breaks.OrderBy(s => s.Start).LastOrDefault().End;

void ShowResult()
{
    Console.WriteLine("");
    Console.WriteLine("");

    foreach (var item in workingTime)
    {
        Console.WriteLine($"Worktime: {item.Start} - {item.End}");
    }
}