chore: add hrynco common library solution

- add the standalone HrynCo.Common solution and projects
- include the shared common library source and tests
- add package metadata and a repo gitignore

HrynCo.Common/Clock.cs

@@ -0,0 +1,40 @@
+namespace HrynCo.Common;
+
+public sealed class Clock : IClock
+{
+    private readonly DateTimeOffset _createdAt;
+    private readonly DateTimeOffset _initialNow;
+    private readonly DateTimeOffset _initialUtcNow;
+
+    public Clock()
+        : this(DateTimeOffset.UtcNow)
+    {
+    }
+
+    public Clock(DateTimeOffset initialUtcNow)
+    {
+        _initialUtcNow = initialUtcNow;
+        _initialNow = initialUtcNow.ToLocalTime();
+        _createdAt = DateTimeOffset.UtcNow;
+    }
+
+    public DateTimeOffset Now => _initialNow + (DateTimeOffset.UtcNow - _createdAt);
+    public DateTimeOffset UtcNow => _initialUtcNow + (DateTimeOffset.UtcNow - _createdAt);
+    public DateOnly Today => DateOnly.FromDateTime(Now.DateTime);
+
+    public DateOnly GetNextDayOfWeek(DayOfWeek dayOfWeek)
+    {
+        int daysToAdd = ((int)dayOfWeek - (int)Today.DayOfWeek + 7) % 7;
+        return Today.AddDays(daysToAdd == 0 ? 7 : daysToAdd);
+    }
+
+    public DateOnly GetLastDayOfMonth()
+    {
+        return new DateOnly(Now.Year, Now.Month, 1).AddMonths(1).AddDays(-1);
+    }
+
+    public DateOnly GetNextMonthStart()
+    {
+        return new DateOnly(Now.Year, Now.Month, 1).AddMonths(1);
+    }
+}