Formatting Exception when using DateTime for PlayerPrefs

Hi i need help, i am getting a FormatException when i try the below codes, it will save datetime and save to playerprefs then on resume will get the difference from the old time and current time, but i am having an Format exception, anyone who knows how to convert these things without errors? thank you

import UnityEngine;
import System.Collections;
import System;

var currentDate : DateTime;
var oldDate : DateTime;
var timex : double;

function Start()

 //Store the current time when it starts
 currentDate = System.DateTime.Now;

//Grab the old time from the playerprefs as a long
var temp : long = Convert.ToInt64(PlayerPrefs.GetString("Time_Data"));

//Convert the old time from binary to a DataTime variable
var oldDate : DateTime = DateTime.FromBinary(temp);
print("oldDate: " + oldDate);

//Use the Subtract method and store the result as a timespan variable
var difference : TimeSpan = currentDate.Subtract(oldDate);
print("Difference: " + difference.TotalSeconds);

timex = timex + difference.TotalSeconds;


function OnApplicationQuit()


//Save the current system time as a string in the playerprefs class
PlayerPrefs.SetString("Time_Data", System.DateTime.Now.ToBinary().ToString());

print("Saving this date to prefs: " + System.DateTime.Now);


@andyblueAMS001 A while ago i was testing with something like this so I am copy pasting this code as it is. SO you may understand it more precisely.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using System;

public class TimeTest : MonoBehaviour
string a;
DateTime da,da1;
int hours, min, sec;
int hoursNow, minNow, secNow;
long secondsBefore, secondsNow,timePassed;
int temp;

void Start()
    temp = PlayerPrefs.GetInt("rated");
    Debug.Log("rated" + temp);
    //Debug.Log(" Time: "+System.DateTime.Now.ToString("HH:mm:ss"));
    da = System.DateTime.Now;
    PlayerPrefs.SetInt("Hours", int.Parse(da.ToString("HH")));
    PlayerPrefs.SetInt("Minutes", int.Parse(da.ToString("mm")));
    PlayerPrefs.SetInt("Seconds", int.Parse(da.ToString("ss")));


    //Debug.Log(" time " + hours+" "+ min+" " + sec);


IEnumerator ChangeTime()
    yield return new WaitForSeconds(10.0f);
    hours = PlayerPrefs.GetInt("Hours");
    min = PlayerPrefs.GetInt("Minutes");
    sec = PlayerPrefs.GetInt("Seconds");
    da1 = System.DateTime.Now;
    secondsBefore = (hours * 60 * 60) + (min * 60) + sec;
    hoursNow = int.Parse(da1.ToString("HH"));
    minNow= int.Parse(da1.ToString("mm"));
    secNow= int.Parse(da1.ToString("ss"));

    secondsNow= (hoursNow * 60 * 60) + (minNow * 60) + secNow;

    timePassed = secondsNow - secondsBefore;
    Debug.Log("Ti9mePassed " + timePassed);