CS1503 Only For Non-Zero Values

Question

I'm getting a compile error CS1503 (cannot convert from typeA to typeB), but only when I give a non-zero value. Why does this not happen when using zero?

For example, only the .AddDays(7 - rangeEnd.DayOfWeek) below gives this error.

                //start on sunday
                var rangeStart = new DateTime(reportingMonth.Year, reportingMonth.Month, 1);
                rangeStart = rangeStart.AddDays(0 - rangeStart.DayOfWeek);

                //end on saturday
                var rangeEnd = new DateTime(reportingMonth.Year, reportingMonth.Month, 1).AddMonths(1).AddDays(-1);
                rangeEnd = rangeEnd.AddDays(7 - rangeEnd.DayOfWeek);

I'm running .net framework 4.7.1.