diff --git a/julian/julian_test.go b/julian/julian_test.go
new file mode 100644
index 0000000..dbe6538
--- /dev/null
+++ b/julian/julian_test.go
@@ -0,0 +1,147 @@
+package julian
+
+import (
+	"fmt"
+	"testing"
+	"time"
+)
+
+func TestMonthLength(t *testing.T) {
+	var tests = []struct {
+		y    int
+		m    time.Month
+		want int
+	}{
+		{2001, time.January, 31},
+		{2001, time.February, 28},
+		{2000, time.February, 29},
+		{2001, time.March, 31},
+		{2001, time.April, 30},
+		{2001, time.May, 31},
+		{2001, time.June, 30},
+		{2001, time.July, 31},
+		{2001, time.August, 31},
+		{2001, time.September, 30},
+		{2001, time.October, 31},
+		{2001, time.November, 30},
+		{2001, time.December, 31},
+	}
+
+	for _, tt := range tests {
+		testname := fmt.Sprintf("%v, %d", tt.m, tt.y)
+		t.Run(testname, func(t *testing.T) {
+			ans := monthLength(tt.y, tt.m)
+			if ans != tt.want {
+				t.Errorf("got %d, want %d", ans, tt.want)
+			}
+		})
+	}
+}
+
+func TestDateDiff(t *testing.T) {
+	var tests = []struct {
+		d    Date
+		days int
+		want Date
+	}{
+		{Date{2001, time.January, 20}, 5, Date{2001, time.January, 25}},
+		{Date{2001, time.February, 28}, 5, Date{2001, time.March, 5}},
+		{Date{2004, time.February, 28}, 5, Date{2004, time.March, 4}},
+	}
+
+	for _, tt := range tests {
+		testname := fmt.Sprintf("%v, %d", tt.d, tt.days)
+		t.Run(testname, func(t *testing.T) {
+			ans := dateDiff(tt.d, tt.days)
+			if ans != tt.want {
+				t.Errorf("got %d, want %d", ans, tt.want)
+			}
+		})
+	}
+}
+
+func TestNextMonth(t *testing.T) {
+	ans1, ans2 := nextMonth(2000, time.December)
+	if ans1 != 2001 || ans2 != time.January {
+		t.Errorf("nextMonth(2000, time.December) = %d, %v; want 2001, time.January", ans1, ans2)
+	}
+}
+
+func TestToTime(t *testing.T) {
+	gDate := func(year int, month time.Month, day int) time.Time {
+		return time.Date(year, month, day, 0, 0, 0, 0, time.UTC)
+	}
+	var tests = []struct {
+		d    Date
+		want time.Time
+	}{
+		{Date{100, time.February, 29}, gDate(100, time.February, 27)},
+		{Date{100, time.March, 1}, gDate(100, time.February, 28)},
+		{Date{100, time.March, 2}, gDate(100, time.March, 1)},
+		{Date{200, time.February, 28}, gDate(200, time.February, 27)},
+		{Date{200, time.February, 29}, gDate(200, time.February, 28)},
+		{Date{200, time.March, 1}, gDate(200, time.March, 1)},
+		{Date{300, time.February, 28}, gDate(300, time.February, 28)},
+		{Date{300, time.February, 29}, gDate(300, time.March, 1)},
+		{Date{300, time.March, 1}, gDate(300, time.March, 2)},
+		{Date{500, time.February, 28}, gDate(500, time.March, 1)},
+		{Date{500, time.February, 29}, gDate(500, time.March, 2)},
+		{Date{500, time.March, 1}, gDate(500, time.March, 3)},
+		{Date{600, time.February, 28}, gDate(600, time.March, 2)},
+		{Date{600, time.February, 29}, gDate(600, time.March, 3)},
+		{Date{600, time.March, 1}, gDate(600, time.March, 4)},
+		{Date{700, time.February, 28}, gDate(700, time.March, 3)},
+		{Date{700, time.February, 29}, gDate(700, time.March, 4)},
+		{Date{700, time.March, 1}, gDate(700, time.March, 5)},
+		{Date{900, time.February, 28}, gDate(900, time.March, 4)},
+		{Date{900, time.February, 29}, gDate(900, time.March, 5)},
+		{Date{900, time.March, 1}, gDate(900, time.March, 6)},
+		{Date{1000, time.February, 28}, gDate(1000, time.March, 5)},
+		{Date{1000, time.February, 29}, gDate(1000, time.March, 6)},
+		{Date{1000, time.March, 1}, gDate(1000, time.March, 7)},
+		{Date{1100, time.February, 28}, gDate(1100, time.March, 6)},
+		{Date{1100, time.February, 29}, gDate(1100, time.March, 7)},
+		{Date{1100, time.March, 1}, gDate(1100, time.March, 8)},
+		{Date{1300, time.February, 28}, gDate(1300, time.March, 7)},
+		{Date{1300, time.February, 29}, gDate(1300, time.March, 8)},
+		{Date{1300, time.March, 1}, gDate(1300, time.March, 9)},
+		{Date{1400, time.February, 28}, gDate(1400, time.March, 8)},
+		{Date{1400, time.February, 29}, gDate(1400, time.March, 9)},
+		{Date{1400, time.March, 1}, gDate(1400, time.March, 10)},
+		{Date{1500, time.February, 28}, gDate(1500, time.March, 9)},
+		{Date{1500, time.February, 29}, gDate(1500, time.March, 10)},
+		{Date{1500, time.March, 1}, gDate(1500, time.March, 11)},
+		{Date{1582, time.October, 4}, gDate(1582, time.October, 14)},
+		{Date{1582, time.October, 5}, gDate(1582, time.October, 15)},
+		{Date{1582, time.October, 6}, gDate(1582, time.October, 16)},
+		{Date{1700, time.February, 18}, gDate(1700, time.February, 28)},
+		{Date{1700, time.February, 19}, gDate(1700, time.March, 1)},
+		{Date{1700, time.February, 28}, gDate(1700, time.March, 10)},
+		{Date{1700, time.February, 29}, gDate(1700, time.March, 11)},
+		{Date{1700, time.March, 1}, gDate(1700, time.March, 12)},
+		{Date{1800, time.February, 17}, gDate(1800, time.February, 28)},
+		{Date{1800, time.February, 18}, gDate(1800, time.March, 1)},
+		{Date{1800, time.February, 28}, gDate(1800, time.March, 11)},
+		{Date{1800, time.February, 29}, gDate(1800, time.March, 12)},
+		{Date{1800, time.March, 1}, gDate(1800, time.March, 13)},
+		{Date{1900, time.February, 16}, gDate(1900, time.February, 28)},
+		{Date{1900, time.February, 17}, gDate(1900, time.March, 1)},
+		{Date{1900, time.February, 28}, gDate(1900, time.March, 12)},
+		{Date{1900, time.February, 29}, gDate(1900, time.March, 13)},
+		{Date{1900, time.March, 1}, gDate(1900, time.March, 14)},
+		{Date{2100, time.February, 15}, gDate(2100, time.February, 28)},
+		{Date{2100, time.February, 16}, gDate(2100, time.March, 1)},
+		{Date{2100, time.February, 28}, gDate(2100, time.March, 13)},
+		{Date{2100, time.February, 29}, gDate(2100, time.March, 14)},
+	}
+
+	for _, tt := range tests {
+		testname := fmt.Sprintf("%v, %v", tt.d, tt.want)
+		t.Run(testname, func(t *testing.T) {
+			ans := ToTime(tt.d)
+			if ans != tt.want {
+				t.Errorf("got %v, want %v", ans, tt.want)
+			}
+		})
+	}
+}