diff --git a/file.go b/file.go
index 1c344087..83e4b5bb 100644
--- a/file.go
+++ b/file.go
@@ -12,7 +12,41 @@ var dirsToSkip = []string{
 	"node_modules",
 }
 
-func minTime(pattern string) (minTime time.Time, err error) {
+func minTime(a, b time.Time) time.Time {
+	if !a.IsZero() && a.Before(b) {
+		return a
+	}
+	return b
+}
+func maxTime(a, b time.Time) time.Time {
+	if a.After(b) {
+		return a
+	}
+	return b
+}
+
+func getPatternsMinTime(patterns []string) (m time.Time, err error) {
+	for _, p := range patterns {
+		mp, err := getPatternMinTime(p)
+		if err != nil {
+			return time.Time{}, err
+		}
+		m = minTime(m, mp)
+	}
+	return
+}
+func getPatternsMaxTime(patterns []string) (m time.Time, err error) {
+	for _, p := range patterns {
+		mp, err := getPatternMaxTime(p)
+		if err != nil {
+			return time.Time{}, err
+		}
+		m = maxTime(m, mp)
+	}
+	return
+}
+
+func getPatternMinTime(pattern string) (minTime time.Time, err error) {
 	files, err := zglob.Glob(pattern)
 	if err != nil {
 		return time.Time{}, err
@@ -32,7 +66,7 @@ func minTime(pattern string) (minTime time.Time, err error) {
 	return
 }
 
-func maxTime(pattern string) (maxTime time.Time, err error) {
+func getPatternMaxTime(pattern string) (maxTime time.Time, err error) {
 	files, err := zglob.Glob(pattern)
 	if err != nil {
 		return time.Time{}, err
diff --git a/task.go b/task.go
index 902470f5..5f5aae17 100644
--- a/task.go
+++ b/task.go
@@ -30,8 +30,8 @@ func init() {
 type Task struct {
 	Cmds      []string
 	Deps      []string
-	Sources   string
-	Generates string
+	Sources   []string
+	Generates []string
 }
 
 type TaskNotFoundError struct {
@@ -104,16 +104,16 @@ func RunTask(name string) error {
 }
 
 func isTaskUpToDate(t *Task) bool {
-	if t.Sources == "" || t.Generates == "" {
+	if len(t.Sources) == 0 || len(t.Generates) == 0 {
 		return false
 	}
 
-	sourcesMaxTime, err := maxTime(t.Sources)
+	sourcesMaxTime, err := getPatternsMaxTime(t.Sources)
 	if err != nil || sourcesMaxTime.IsZero() {
 		return false
 	}
 
-	generatesMinTime, err := minTime(t.Generates)
+	generatesMinTime, err := getPatternsMinTime(t.Generates)
 	if err != nil || generatesMinTime.IsZero() {
 		return false
 	}