feat:add betweenSeconds func (#102)

Co-authored-by: leitao <leitao@kezaihui.com>

datetime/datetime_test.go

@@ -1,6 +1,7 @@
 package datetime
 
 import (
+	"fmt"
 	"testing"
 	"time"
 
@@ -20,6 +21,12 @@ func TestAddYear(t *testing.T) {
 	assert.Equal(float64(-8760), diff2.Hours())
 }
 
+func TestBetweenSeconds(t *testing.T) {
+	now := time.Now()
+	target := AddDay(now, 1)
+	fmt.Println(BetweenSeconds(now, target))
+}
+
 func TestAddDay(t *testing.T) {
 	assert := internal.NewAssert(t, "TestAddDay")