You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: helper.go
    path of file: ./repos/evcc/core/planner
    
    the code of the file until where you have to start completion: package planner

import (
	"time"

	"github.com/evcc-io/evcc/api"
)

// Start returns the earliest slot's start time
func Start(plan api.Rates) time.Time {
	var start 
    
func SlotAt(time time.Time, plan api.Rates) api.Rate {
	for _, slot := range plan {
		if (slot.Start.Before(time) || slot.Start.Equal(time)) && slot.End.After(time) {
			return slot
		}
	}
	return api.Rate{}
}