diff --git a/internal/timeutil/timeutil.go b/internal/timeutil/timeutil.go
new file mode 100644
index 0000000..65691ad
--- /dev/null
+++ b/internal/timeutil/timeutil.go
@@ -0,0 +1,38 @@
+// Package timeutil exports functions and types related to time and date.
+package timeutil
+
+import "time"
+
+// A Clock is an object that can tell you the current time.
+//
+// This interface allows decoupling code that uses time from the code that creates
+// a point in time. You can use this to your advantage by injecting Clocks into interfaces
+// rather than having implementations call time.Now() directly.
+//
+// Use RealClock() in production.
+// Use SimulatedClock() in test.
+type Clock interface {
+	Now() time.Time
+}
+
+func NewRealClock() Clock { return &realTimeClock{} }
+
+type realTimeClock struct{}
+
+func (_ *realTimeClock) Now() time.Time { return time.Now() }
+
+// A SimulatedClock is a concrete Clock implementation that doesn't "tick" on its own.
+// Time is advanced by explicit call to the AdvanceTime() or SetTime() functions.
+type SimulatedClock struct {
+	t time.Time
+}
+
+func NewSimulatedClock(t time.Time) *SimulatedClock {
+	return &SimulatedClock{t}
+}
+
+func (c *SimulatedClock) Now() time.Time { return c.t }
+
+func (c *SimulatedClock) SetTime(t time.Time) { c.t = t }
+
+func (c *SimulatedClock) AdvanceTime(d time.Duration) { c.t.Add(d) }