diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..f3e8252
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,5 @@
+module github.com/dontlaugh/lilrange
+
+go 1.12
+
+require github.com/stretchr/testify v1.3.0
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..4347755
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,7 @@
+github.com/davecgh/go-spew v1.1.0 h1:ZDRjVQ15GmhC3fiQ8ni8+OwkZQO4DARzQgrnXU1Liz8=
+github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
+github.com/stretchr/testify v1.3.0 h1:TivCn/peBQ7UY8ooIcPgZFpTNSz0Q2U6UrFlUfqbe0Q=
+github.com/stretchr/testify v1.3.0/go.mod h1:M5WIy9Dh21IEIfnGCwXGc5bZfKNJtfHm1UVUgZn+9EI=
diff --git a/lilrange.go b/lilrange.go
new file mode 100644
index 0000000..98d9fcd
--- /dev/null
+++ b/lilrange.go
@@ -0,0 +1,134 @@
+package lilrange
+
+import (
+	"errors"
+	"fmt"
+	"strconv"
+	"strings"
+	"time"
+)
+
+type Range struct {
+	Start    time.Time
+	End      time.Time
+	Duration time.Duration
+	repr     string
+}
+
+func (t Range) Within(instant time.Time) bool {
+	return instant.After(t.Start) && instant.Before(t.End)
+}
+
+func (r Range) Next() Range {
+	return Range{
+		r.Start.Add(24 * time.Hour),
+		r.End.Add(24 * time.Hour),
+		r.Duration,
+		r.repr,
+	}
+}
+
+func Parse(s string) (*Range, error) {
+	var tr Range
+	splitted := strings.Split(s, "-")
+	if len(splitted) != 2 {
+		return nil, fmt.Errorf("invalid time range value: %s", s)
+	}
+	startHr, startMin, err := extractAndValidate(splitted[0])
+	if err != nil {
+		return nil, err
+	}
+	endHr, endMin, err := extractAndValidate(splitted[1])
+	if err != nil {
+		return nil, err
+	}
+	// 0. determine duration
+	dur, _ := CalculateDurationMinutes(startHr, startMin, endHr, endMin)
+	// 1. determine if "today's start" has happened
+	now := time.Now()
+	nowYear, nowMonth, nowDate := now.Date()
+	todaysStart := time.Date(nowYear, nowMonth, nowDate, startHr, startMin, 0, 0, time.UTC)
+	endTime := todaysStart.Add(time.Minute * time.Duration(dur))
+
+	if now.After(endTime) {
+		// today's range has ended. Our default behavior is to return the next
+		// range, e.g. tomorrow's
+		tr.Start = todaysStart.Add(time.Hour * 24)
+		tr.End = endTime.Add(time.Hour * 24)
+	} else {
+		// Today's range has not ended. We are either a) inside this range, or
+		// b) it is in the future and has not started. Either way, assign the
+		// computed times and let the caller decide what to do.
+		tr.Start = todaysStart
+		tr.End = endTime
+	}
+	tr.Duration = time.Minute * time.Duration(dur)
+	tr.repr = s
+	return &tr, nil
+}
+
+func CalculateDurationMinutes(startHr, startMin, endHr, endMin int) (int, bool) {
+	if !between(startHr, 0, 23) || !between(endHr, 0, 23) || !between(startMin, 0, 59) || !between(endMin, 0, 59) {
+		panic("invalid values for hours and/or minutes")
+	}
+	var crossesMidnight bool
+	if startHr > endHr {
+		crossesMidnight = true
+	}
+	var duration int
+	if crossesMidnight {
+		minUntilMidnight := ((24 - startHr) * 60) - startMin
+		minAfterMidnight := (endHr * 60) + endMin
+		duration = minUntilMidnight + minAfterMidnight
+	} else {
+		duration = ((endHr - startHr) * 60) - startMin + endMin
+	}
+	return duration, crossesMidnight
+}
+
+func validRune(r rune) bool {
+	valid := []rune{'0', '1', '2', '3', '4', '5', '6', '7', '8', '9'}
+	for _, x := range valid {
+		if x == r {
+			return true
+		}
+	}
+	return false
+}
+
+func extractAndValidate(s string) (int, int, error) {
+	runes := []rune(s)
+	if len(runes) != 4 {
+		return -1, -1, fmt.Errorf("invalid time component: %s", s)
+	}
+	for pos, char := range s {
+		// TODO make this a comparison to range of ASCII bytes rather than another loop
+		if !validRune(char) {
+			return -1, -1, fmt.Errorf("invalid char at position %v: %c [%v]",
+				pos, char, char)
+
+		}
+	}
+	hr, min := string(runes[0:2]), string(runes[2:4])
+	// We've asserted that our chars are ASCII 0-9, so Atoi should never fail.
+	hrInt, err := strconv.Atoi(hr)
+	if err != nil {
+		panic("how is this possible")
+	}
+	minInt, err := strconv.Atoi(min)
+	if err != nil {
+		panic("universe is broken, sorry")
+	}
+	if !between(hrInt, 0, 23) {
+		return -1, -1, errors.New("hour value must be between 0 and 23")
+	}
+	if !between(minInt, 0, 59) {
+		return -1, -1, errors.New("minute value must be between 0 and 59")
+	}
+	return hrInt, minInt, nil
+}
+
+// x is between this and that
+func between(x, this, that int) bool {
+	return x >= this && x <= that
+}
diff --git a/lilrange_test.go b/lilrange_test.go
new file mode 100644
index 0000000..79d1486
--- /dev/null
+++ b/lilrange_test.go
@@ -0,0 +1,113 @@
+package lilrange
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestParse(t *testing.T) {
+	type testCase struct {
+		input    string
+		expected *Range
+	}
+	var invalidInputs []testCase = []testCase{
+		// invalid inputs, so we expect nil
+		{"0300", nil},
+		{"0300-000", nil},
+		{"300-000", nil},
+		{"-000", nil},
+		{"", nil},
+		{"-", nil},
+		{"-0300", nil},
+		{"-0", nil},
+		{"0030-0", nil},
+	}
+
+	for _, c := range invalidInputs {
+		actual, _ := Parse(c.input)
+		assert.EqualValues(t, c.expected, actual)
+	}
+
+	type durationTest struct {
+		input    string
+		expected time.Duration
+	}
+	var durationTests []durationTest = []durationTest{
+		// valid inputs, and we assert on the durations we compute
+		{"0100-0200", 1 * time.Hour},
+		{"0000-0002", 2 * time.Minute},
+		{"2359-0000", 1 * time.Minute},
+		{"2359-0001", 2 * time.Minute},
+		{"2259-0001", 62 * time.Minute},
+	}
+
+	for _, dt := range durationTests {
+		actual, err := Parse(dt.input)
+		if err != nil {
+			t.Errorf("duration test fail: %v", err)
+		}
+		assert.EqualValues(t, dt.expected, actual.Duration)
+	}
+}
+
+func TestExtractAndValidate(t *testing.T) {
+	type testCase struct {
+		input    string
+		expected []int
+	}
+	var cases = []testCase{
+		// invalid cases
+		{"0", []int{-1, -1}},
+		{"223", []int{-1, -1}},
+		{"2460", []int{-1, -1}},
+		{"2460", []int{-1, -1}},
+		{"0260", []int{-1, -1}},
+		{"9901", []int{-1, -1}},
+		{"12.1", []int{-1, -1}},
+		{"12:1", []int{-1, -1}},
+		{"1:21", []int{-1, -1}},
+		{"1:21", []int{-1, -1}},
+		{"1/21", []int{-1, -1}},
+		// valid cases
+		{"2359", []int{23, 59}},
+		{"0000", []int{0, 0}},
+		{"0001", []int{0, 1}},
+		{"1111", []int{11, 11}},
+		{"0223", []int{2, 23}},
+	}
+
+	for _, c := range cases {
+		hr, min, _ := extractAndValidate(c.input)
+		assert.EqualValues(t, c.expected, []int{hr, min})
+	}
+}
+
+func TestCalculateDurationMinutes(t *testing.T) {
+
+	type testCase struct {
+		// inputs: startHr, startMin, endHr, endMin
+		inputs   []int
+		expected int
+	}
+	var cases = []testCase{
+		{[]int{0, 0, 1, 0}, 60},
+		{[]int{1, 0, 2, 0}, 60},
+		{[]int{23, 0, 1, 0}, 120},
+		{[]int{23, 15, 1, 0}, 105},
+		{[]int{23, 15, 1, 10}, 115},
+	}
+
+	for _, c := range cases {
+		dur, _ := CalculateDurationMinutes(c.inputs[0], c.inputs[1],
+			c.inputs[2], c.inputs[3])
+		assert.EqualValues(t, c.expected, dur)
+	}
+	// Assert that invalid inputs panic.
+	assert.Panics(t, func() { CalculateDurationMinutes(-1, 0, 0, 0) })
+	assert.Panics(t, func() { CalculateDurationMinutes(25, 0, 0, 0) })
+	assert.Panics(t, func() { CalculateDurationMinutes(0, 0, 25, 0) })
+	assert.Panics(t, func() { CalculateDurationMinutes(0, 69, 0, 0) })
+	assert.Panics(t, func() { CalculateDurationMinutes(0, 0, 0, 69) })
+}