diff --git a/README.md b/README.md
index 3116ba9..4213e8e 100644
--- a/README.md
+++ b/README.md
@@ -12,9 +12,9 @@ This library operates off Period interfaces, which contain the following:
 
 ```
 type Period interface {
-        GetStartTime() time.Time
-        GetEndTime() time.Time
-        GetIdentifier() string
+        GetStartTime()  time.Time   // Inclusive start time
+        GetEndTime()    time.Time   // Exclusive end time ("expiration time")
+        GetIdentifier() string 
 }
 ```
 
diff --git a/msp/changeover.go b/msp/changeover.go
index 268e3dc..24a735a 100644
--- a/msp/changeover.go
+++ b/msp/changeover.go
@@ -44,7 +44,7 @@ func GetNextChangeOver(t time.Time, periods ...Period) (ts time.Time, err error)
 			return ts, nil
 		}
 	}
-	return time.Unix(0, 0), ErrNoNextChangeover
+	return time.Time{}, ErrNoNextChangeover
 }
 
 func FlattenPeriods(periods ...Period) (ids []string) {
diff --git a/msp/changeover_test.go b/msp/changeover_test.go
new file mode 100644
index 0000000..25fcf7b
--- /dev/null
+++ b/msp/changeover_test.go
@@ -0,0 +1,304 @@
+package msp
+
+import (
+	"fmt"
+	"testing"
+	"time"
+)
+
+func slicesEqual[K comparable](x []K, y []K) bool {
+	if len(x) != len(y) {
+		return false
+	}
+	for i, w := range x {
+		if w != y[i] {
+			return false
+		}
+	}
+	return true
+}
+
+func TestGetChangeOvers(t *testing.T) {
+	// use a static timestamp to make sure tests don't fail on slower systems or during a process pause
+	now := time.Now()
+	testCases := []struct {
+		ts      time.Time
+		testID  string
+		result  []time.Time
+		periods []Period
+	}{{testID: "No choices",
+		ts:      now,
+		result:  []time.Time{},
+		periods: []Period{}},
+		{testID: "Two Choices, shorter is second",
+			ts:     now,
+			result: []time.Time{now.Add(-5 * time.Minute), now.Add(-2 * time.Minute), now.Add(time.Minute)},
+			periods: []Period{TimeWindow{StartTime: now.Add(-5 * time.Minute),
+				EndTime:    now.Add(time.Minute),
+				Identifier: "A"},
+				TimeWindow{StartTime: now.Add(-2 * time.Minute),
+					EndTime:    now.Add(time.Minute),
+					Identifier: "B"}}},
+		{testID: "Two Choices, one is a year, other a minute",
+			ts:     now,
+			result: []time.Time{now.Add(-1 * time.Hour * 24 * 365), now.Add(-5 * time.Minute), now.Add(time.Minute)},
+			periods: []Period{TimeWindow{StartTime: now.Add(-1 * time.Hour * 24 * 365),
+				EndTime:    now.Add(time.Minute),
+				Identifier: "A"},
+				TimeWindow{StartTime: now.Add(-5 * time.Minute),
+					EndTime:    now.Add(time.Minute),
+					Identifier: "B"}}},
+
+		{testID: "Two Choices, shorter is first",
+			ts:     now,
+			result: []time.Time{now.Add(-5 * time.Minute), now.Add(-2 * time.Minute), now.Add(time.Minute)},
+			periods: []Period{TimeWindow{StartTime: now.Add(-2 * time.Minute),
+				EndTime:    now.Add(time.Minute),
+				Identifier: "A"},
+				TimeWindow{StartTime: now.Add(-5 * time.Minute),
+					EndTime:    now.Add(time.Minute),
+					Identifier: "B"}}},
+		{testID: "Two Choices, one in the past",
+			ts:     now,
+			result: []time.Time{now.Add(-2 * time.Minute), now.Add(-time.Minute), now.Add(time.Minute)},
+			periods: []Period{TimeWindow{StartTime: now.Add(-time.Minute),
+				EndTime:    now.Add(time.Minute),
+				Identifier: "A"},
+				TimeWindow{StartTime: now.Add(-2 * time.Minute),
+					EndTime:    now.Add(-time.Minute),
+					Identifier: "B"}}},
+		{testID: "Two Choices, one invalid",
+			ts:     now,
+			result: []time.Time{now.Add(-2 * time.Minute), now.Add(time.Minute)},
+			periods: []Period{TimeWindow{StartTime: now.Add(time.Minute),
+				EndTime:    now.Add(-time.Minute),
+				Identifier: "A"},
+				TimeWindow{StartTime: now.Add(-2 * time.Minute),
+					EndTime:    now.Add(time.Minute),
+					Identifier: "B"}}},
+		{testID: "Two Choices, Identical periods",
+			ts:     now,
+			result: []time.Time{now.Add(-time.Minute), now.Add(time.Minute)},
+			periods: []Period{TimeWindow{StartTime: now.Add(-time.Minute),
+				EndTime:    now.Add(time.Minute),
+				Identifier: "A"},
+				TimeWindow{StartTime: now.Add(-time.Minute),
+					EndTime:    now.Add(time.Minute),
+					Identifier: "B"}}},
+		{testID: "One choice",
+			ts:     now,
+			result: []time.Time{now.Add(-time.Minute), now.Add(time.Minute)},
+			periods: []Period{TimeWindow{StartTime: now.Add(-time.Minute),
+				EndTime:    now.Add(time.Minute),
+				Identifier: "A"}}}}
+	for _, tc := range testCases {
+		t.Run(fmt.Sprintf("%s", tc.testID), func(t *testing.T) {
+			changeovers := GetChangeOvers(tc.periods...)
+			if !slicesEqual(changeovers, tc.result) {
+				t.Errorf("Expected %v but got %v", tc.result, changeovers)
+			}
+
+		})
+	}
+}
+func TestFlattenPeriods(t *testing.T) {
+	// use a static timestamp to make sure tests don't fail on slower systems or during a process pause
+	now := time.Now()
+	testCases := []struct {
+		ts      time.Time
+		testID  string
+		result  []string
+		err     error
+		periods []Period
+	}{{testID: "No choices",
+		ts:      now,
+		result:  []string{},
+		err:     ErrNoValidPeriods,
+		periods: []Period{}},
+		{testID: "Two Choices, shorter is second",
+			ts:     now,
+			result: []string{"A", "B"},
+			err:    nil,
+			periods: []Period{TimeWindow{StartTime: now.Add(-5 * time.Minute),
+				EndTime:    now.Add(time.Minute),
+				Identifier: "A"},
+				TimeWindow{StartTime: now.Add(-2 * time.Minute),
+					EndTime:    now.Add(time.Minute),
+					Identifier: "B"}}},
+		{testID: "Two Choices, one is a year, other a minute",
+			ts:     now,
+			result: []string{"A", "B"},
+			err:    nil,
+			periods: []Period{TimeWindow{StartTime: now.Add(-1 * time.Hour * 24 * 365),
+				EndTime:    now.Add(time.Minute),
+				Identifier: "A"},
+				TimeWindow{StartTime: now.Add(-5 * time.Minute),
+					EndTime:    now.Add(time.Minute),
+					Identifier: "B"}}},
+
+		{testID: "Two Choices, shorter is first",
+			ts:     now,
+			result: []string{"B", "A"},
+			err:    nil,
+			periods: []Period{TimeWindow{StartTime: now.Add(-2 * time.Minute),
+				EndTime:    now.Add(time.Minute),
+				Identifier: "A"},
+				TimeWindow{StartTime: now.Add(-5 * time.Minute),
+					EndTime:    now.Add(time.Minute),
+					Identifier: "B"}}},
+		{testID: "Two Choices, one in the past",
+			ts:     now,
+			result: []string{"B", "A"},
+			err:    nil,
+			periods: []Period{TimeWindow{StartTime: now.Add(-time.Minute),
+				EndTime:    now.Add(time.Minute),
+				Identifier: "A"},
+				TimeWindow{StartTime: now.Add(-2 * time.Minute),
+					EndTime:    now.Add(-time.Minute),
+					Identifier: "B"}}},
+		{testID: "Two Choices, one invalid",
+			ts:     now,
+			result: []string{"B"},
+			err:    nil,
+			periods: []Period{TimeWindow{StartTime: now.Add(time.Minute),
+				EndTime:    now.Add(-time.Minute),
+				Identifier: "A"},
+				TimeWindow{StartTime: now.Add(-2 * time.Minute),
+					EndTime:    now.Add(time.Minute),
+					Identifier: "B"}}},
+		{testID: "Two Choices, Identical periods",
+			ts:     now,
+			result: []string{"B"},
+			err:    nil,
+			periods: []Period{TimeWindow{StartTime: now.Add(-time.Minute),
+				EndTime:    now.Add(time.Minute),
+				Identifier: "A"},
+				TimeWindow{StartTime: now.Add(-time.Minute),
+					EndTime:    now.Add(time.Minute),
+					Identifier: "B"}}},
+		{testID: "Triple Nested Periods",
+			ts:     now,
+			result: []string{"A", "B", "C", "B", "A"},
+			err:    nil,
+			periods: []Period{TimeWindow{StartTime: now.Add(-15 * time.Minute),
+				EndTime:    now.Add(15 * time.Minute),
+				Identifier: "A"},
+				TimeWindow{StartTime: now.Add(-5 * time.Minute),
+					EndTime:    now.Add(5 * time.Minute),
+					Identifier: "C"},
+				TimeWindow{StartTime: now.Add(-10 * time.Minute),
+					EndTime:    now.Add(10 * time.Minute),
+					Identifier: "B"}}},
+		{testID: "One choice",
+			ts:     now,
+			result: []string{"A"},
+			err:    nil,
+			periods: []Period{TimeWindow{StartTime: now.Add(-time.Minute),
+				EndTime:    now.Add(time.Minute),
+				Identifier: "A"}}}}
+
+	for _, tc := range testCases {
+		t.Run(fmt.Sprintf("%s", tc.testID), func(t *testing.T) {
+			changeovers := FlattenPeriods(tc.periods...)
+			if !slicesEqual(changeovers, tc.result) {
+				t.Errorf("Expected %v but got %v", tc.result, changeovers)
+			}
+
+		})
+	}
+}
+func TestGetNextChangeOver(t *testing.T) {
+	// use a static timestamp to make sure tests don't fail on slower systems or during a process pause
+	now := time.Now()
+	testCases := []struct {
+		ts      time.Time
+		testID  string
+		result  time.Time
+		err     error
+		periods []Period
+	}{{testID: "No choices",
+		ts:      now,
+		result:  time.Time{},
+		err:     ErrNoNextChangeover,
+		periods: []Period{}},
+		{testID: "Two Choices, shorter is second",
+			ts:     now,
+			result: now.Add(time.Minute),
+			err:    nil,
+			periods: []Period{TimeWindow{StartTime: now.Add(-5 * time.Minute),
+				EndTime:    now.Add(time.Minute),
+				Identifier: "A"},
+				TimeWindow{StartTime: now.Add(-2 * time.Minute),
+					EndTime:    now.Add(time.Minute),
+					Identifier: "B"}}},
+		{testID: "Two Choices, one is a year, other a minute",
+			ts:     now,
+			result: now.Add(time.Minute),
+			err:    nil,
+			periods: []Period{TimeWindow{StartTime: now.Add(-1 * time.Hour * 24 * 365),
+				EndTime:    now.Add(time.Minute),
+				Identifier: "A"},
+				TimeWindow{StartTime: now.Add(-5 * time.Minute),
+					EndTime:    now.Add(time.Minute),
+					Identifier: "B"}}},
+
+		{testID: "Two Choices, shorter is first",
+			ts:     now,
+			result: now.Add(time.Minute),
+			err:    nil,
+			periods: []Period{TimeWindow{StartTime: now.Add(-2 * time.Minute),
+				EndTime:    now.Add(time.Minute),
+				Identifier: "A"},
+				TimeWindow{StartTime: now.Add(-5 * time.Minute),
+					EndTime:    now.Add(time.Minute),
+					Identifier: "B"}}},
+		{testID: "Two Choices, one in the past",
+			ts:     now,
+			result: now.Add(time.Minute),
+			err:    nil,
+			periods: []Period{TimeWindow{StartTime: now.Add(-time.Minute),
+				EndTime:    now.Add(time.Minute),
+				Identifier: "A"},
+				TimeWindow{StartTime: now.Add(-2 * time.Minute),
+					EndTime:    now.Add(-time.Minute),
+					Identifier: "B"}}},
+		{testID: "Two Choices, one invalid",
+			ts:     now,
+			result: now.Add(time.Minute),
+			err:    nil,
+			periods: []Period{TimeWindow{StartTime: now.Add(time.Minute),
+				EndTime:    now.Add(-time.Minute),
+				Identifier: "A"},
+				TimeWindow{StartTime: now.Add(-2 * time.Minute),
+					EndTime:    now.Add(time.Minute),
+					Identifier: "B"}}},
+		{testID: "Two Choices, Identical periods",
+			ts:     now,
+			result: now.Add(time.Minute),
+			err:    nil,
+			periods: []Period{TimeWindow{StartTime: now.Add(-time.Minute),
+				EndTime:    now.Add(time.Minute),
+				Identifier: "A"},
+				TimeWindow{StartTime: now.Add(-time.Minute),
+					EndTime:    now.Add(time.Minute),
+					Identifier: "B"}}},
+		{testID: "One choice",
+			ts:     now,
+			result: now.Add(time.Minute),
+			err:    nil,
+			periods: []Period{TimeWindow{StartTime: now.Add(-time.Minute),
+				EndTime:    now.Add(time.Minute),
+				Identifier: "A"}}}}
+	for _, tc := range testCases {
+		t.Run(fmt.Sprintf("%s", tc.testID), func(t *testing.T) {
+			ts, err := GetNextChangeOver(now, tc.periods...)
+			if tc.err != err {
+				t.Errorf("Error %v does not match expected %v", tc.err, err)
+			}
+			if ts != tc.result {
+				t.Errorf("Got %v but expected %v", ts, tc.result)
+			}
+
+		})
+	}
+}
diff --git a/msp/errors.go b/msp/errors.go
index ba5aa9b..3733175 100644
--- a/msp/errors.go
+++ b/msp/errors.go
@@ -10,5 +10,5 @@ var (
 	// ErrNoValidPeriods occurs when an empty set of periods is passed or when ll periods are invalid
 	ErrNoValidPeriods = errors.New("error: no valid periods available")
 	// ErrNoNextChangeover occurs when GetNextChangeover is called but there are no changeovers after t
-	ErrNoNextChangeover = errors.New("error: no valid periods available")
+	ErrNoNextChangeover = errors.New("error: no valid changeovers available")
 )
diff --git a/msp/msp.go b/msp/msp.go
index 8f9e7d9..93d7b50 100644
--- a/msp/msp.go
+++ b/msp/msp.go
@@ -61,7 +61,9 @@ func GetDuration(start time.Time, end time.Time) (dur time.Duration, err error)
 func ValidTimePeriods(ts time.Time, periods ...Period) []Period {
 	var valid []Period
 	for _, p := range periods {
-		if p.GetStartTime().Before(ts) && p.GetEndTime().After(ts) {
+		start := p.GetStartTime()
+		end := p.GetEndTime()
+		if (start.Before(ts) || start.Equal(ts)) && (end.After(ts)) {
 			valid = append(valid, p)
 		}
 	}