diff --git a/src/cron/expression.v b/src/cron/expression.v
index 8781900..a29bf53 100644
--- a/src/cron/expression.v
+++ b/src/cron/expression.v
@@ -1,6 +1,7 @@
 module cron
 
 import math
+import time
 
 struct CronExpression {
 	minutes []u32
@@ -8,7 +9,32 @@ struct CronExpression {
 	days    []u32
 }
 
-// parse_range parses a given string into a range of integers, if possible.
+// next calculates the earliest time this cron expression is valid.
+pub fn (ce &CronExpression) next(ref &time.Time) time.Time {
+	res := time.Time{}
+
+	mut day := 0
+	mut hour := 0
+	mut minute := 0
+
+	// Find the next minute
+	// If ref.minute is greater than 
+	if ref.minute >= ce.minutes[ce.minutes.len - 1] || ref.minute < ce.minutes[0] {
+		minute = ce.minutes[0]
+	}else{
+		for i in 0..ce.minutes.len {
+			if ce.minutes[i] > ref.minute {
+				minute = ce.minutes[i]
+				break
+			}
+		}
+	}
+
+	return res
+}
+
+// parse_range parses a given string into a range of sorted integers, if
+// possible.
 fn parse_range(s string, min u32, max u32) ?[]u32 {
 	mut out := []u32{}
 	mut start := min
@@ -29,6 +55,10 @@ fn parse_range(s string, min u32, max u32) ?[]u32 {
 		}
 	}
 
+	if interval == 0 {
+		return []
+	}
+
 	for start <= max {
 		out << start
 		start += interval
diff --git a/src/cron/expression_test.v b/src/cron/expression_test.v
index 6d293c5..abd5e5f 100644
--- a/src/cron/expression_test.v
+++ b/src/cron/expression_test.v
@@ -1,5 +1,6 @@
 module cron
 
+// =====parse_range=====
 fn test_parse_star_range() ? {
 	assert parse_range('*', 0, 5) ? == [u32(0), 1, 2, 3, 4, 5]
 }
@@ -24,6 +25,10 @@ fn test_parse_step_star_too_large() ? {
 	assert parse_range('*/21', 0, 20) ? == [u32(0)]
 }
 
+fn test_parse_step_zero() ? {
+	assert parse_range('*/0', 0, 20) ? == []
+}
+
 fn test_parse_step_number() ? {
 	assert parse_range('5/4', 0, 20) ? == [u32(5), 9, 13, 17]
 }
@@ -35,3 +40,5 @@ fn test_parse_step_number_too_large() ? {
 fn test_parse_step_number_too_small() ? {
 	assert parse_range('2/4', 5, 10) ? == [u32(5), 9]
 }
+
+