Subtracting a time difference from an int?

Hey! I have a question, I have been trying to make a timer that keeps going down while the application is closed (using System.DateTime) here is my script.

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 using System;
 public class Craft : MonoBehaviour {
     DateTime currentDate;
     DateTime oldDate;
 
     public float tinpaneltimer;
     public bool tinpaneltimerOn = false;
 
     public string saveLocation;
     public static Craft instance;
 
 
     public Text CraftSteelButtonText;
     public Text CraftElectrumButtonText;
     public Text CraftTinPanelButtonText;
     public int Iron;
     public int Coal;
     public int Steel;
 
     public int Silver;
     public int GoldOre;
     public int Electrum;
 
     public int Diff;
 
     public int Tin;
     public int TinPanel;
 
 
 
     public int XP;
     public int XPCap;
     public int Level;
     public int XPCapMulitplier;
 
     public Text SteelAmount;
     public Text ElectrumAmount;
     public Text TinPanelAmount;
 
     public Text SlotSteelText;
     public Text SlotElectrumText;
     public Text SlotTinPanelText;
 
     public bool CraftSteelOn = false;
     public bool CraftElectrumOn = false;
     public bool CraftTinPanelOn = false;
 
 
     public Button CraftSteelButton;
     public Image CraftSteelButtonImage;
     public Image CraftSteelButtonImageRight;
 
     public Button CraftElectrumButton;
     public Image CraftElectrumButtonImage;
     public Image CraftElectrumButtonImageRight;
 
     public Button CraftTinPanelButton;
     public Image CraftTinPanelButtonImage;
     public Image CraftTinPanelButtonImageRight;
 
     public Image Panel1;
     public Image Panel2;
     public Image Panel3;
 
     public Image LevelBar;
 
     public AudioSource Audio;
 
 
     void Start()
     {
         CheckDate ();
         if (tinpaneltimer == 0) {
 
             tinpaneltimer = 120;
         }
         if (tinpaneltimerOn == true) {
             
             tinpaneltimer -= instance.CheckDate ();
         }
 
             
     }
 
     void Awake()
     {
         
         instance = this;
 
         saveLocation = "lastSavedDate1";
 
     }
 
     public float CheckDate()
     {
         currentDate = System.DateTime.Now;
         string tempString = PlayerPrefs.GetString (saveLocation, "1");
         long templong = Convert.ToInt64 (tempString);
         DateTime oldDate = DateTime.FromBinary (templong);
         print ("Old Date: " + oldDate);
         TimeSpan difference = currentDate.Subtract (oldDate);

         **tinpaneltimer = (120 - difference);**  //Here is where im trying to figure it out

         print ("Difference: " + difference);
         return (float)difference.TotalSeconds;
     }
 
     public void SaveDate()
     {
         PlayerPrefs.SetString (saveLocation, System.DateTime.Now.ToBinary ().ToString ());
         print ("Saving this date to player prefs: " + System.DateTime.Now);
     }
 
     public void ExitButton()
     {
         Panel1.gameObject.SetActive (false);
         Panel2.gameObject.SetActive (false);
         Panel3.gameObject.SetActive (false);
         LevelBar.gameObject.SetActive (true);
     }
 
     public void ClickSlot1()
     {
         Panel1.gameObject.SetActive (true);
         LevelBar.gameObject.SetActive (false);
     }
     public void ClickSlot2()
     {
         Panel2.gameObject.SetActive (true);
         LevelBar.gameObject.SetActive (false);
     }
     public void ClickSlot3()
     {
         Panel3.gameObject.SetActive (true);
         LevelBar.gameObject.SetActive (false);
     }
         
 
 
 
     void Update()
     {
         
         if (tinpaneltimer == 0) {
             StartC3 ();
         }
         if (tinpaneltimerOn == true) {
             CheckDate ();
             tinpaneltimer -= Time.deltaTime;
         }
 
         //Maybe either have a long time for bigger items or just not and have same craft time.
         SlotSteelText.text = "" + Steel;
         SlotElectrumText.text = "" + Electrum;
         SlotTinPanelText.text = "" + TinPanel;
         if (CraftSteelOn == true) {
             
             CraftSteelButtonImage.fillAmount += 1f / 2f * Time.deltaTime;
             CraftSteelButtonImageRight.fillAmount += 1f / 2f * Time.deltaTime;
         }
         if (CraftElectrumOn == true) {
 
             CraftElectrumButtonImage.fillAmount += 1f / 2f * Time.deltaTime;
             CraftElectrumButtonImageRight.fillAmount += 1f / 2f * Time.deltaTime;
         }
         if (CraftTinPanelOn == true) {
 
             CraftTinPanelButtonImage.fillAmount += 1f / (120 - tinpaneltimer) * Time.deltaTime;
             CraftTinPanelButtonImageRight.fillAmount += 1f / (120 - tinpaneltimer) * Time.deltaTime;
         }
         //Store Date.Time when you start the craft and keep checking it until the subtraction is equal to variable
         SteelAmount.text = "" + Steel;
         ElectrumAmount.text = "" + Electrum;
         TinPanelAmount.text = "" + TinPanel;
         XP = PlayerPrefs.GetInt ("XP");
         XPCap = PlayerPrefs.GetInt ("XPCap");
         Level = PlayerPrefs.GetInt ("Level");
         XPCapMulitplier = PlayerPrefs.GetInt ("XPCapMultiplier");
 
         Iron = PlayerPrefs.GetInt ("Iron");
         Coal = PlayerPrefs.GetInt ("Coal");
         Steel = PlayerPrefs.GetInt ("Steel");
 
         Silver = PlayerPrefs.GetInt ("Silver");
         GoldOre = PlayerPrefs.GetInt ("GoldOre");
         Electrum = PlayerPrefs.GetInt ("Electrum");
         TinPanel = PlayerPrefs.GetInt ("TinPanel");
         Tin = PlayerPrefs.GetInt ("Tin");
 
 
     }
     public void CraftSteel()
     {
         
         if (Iron >= 1 && Coal >= 1) {
             Audio.Play ();
             CraftSteelOn = true;
         
             XP = XP + 20;
             PlayerPrefs.SetInt ("Level", Level);
             PlayerPrefs.SetInt ("XPCap", XPCap);
             PlayerPrefs.SetInt("XP", XP);
             PlayerPrefs.SetInt ("XPCapMultiplier", XPCapMulitplier);
 
             StartC ();
         }
         if (Iron <= 1 || Coal <= 1) {
             CraftSteelButtonText.text = "CANT AFFORD";
     
         }
     }
     public void CraftElectrum()
     {
 
         if (Silver >= 1 && GoldOre >= 1) {
             Audio.Play ();
             CraftElectrumOn = true;
 
             XP = XP + 20;
             PlayerPrefs.SetInt ("Level", Level);
             PlayerPrefs.SetInt ("XPCap", XPCap);
             PlayerPrefs.SetInt("XP", XP);
             PlayerPrefs.SetInt ("XPCapMultiplier", XPCapMulitplier);
 
             StartC2 ();
         }
         if (Silver <= 1 || GoldOre <= 1) {
             CraftElectrumButtonText.text = "CANT AFFORD";
 
         }
     }
     public void CraftTinPanel()
     {
 
         if (Tin >= 4) {
             
             SaveDate ();
             tinpaneltimerOn = true;
             Audio.Play ();
             CraftTinPanelOn = true;
 
 
             XP = XP + 13;
             PlayerPrefs.SetInt ("Level", Level);
             PlayerPrefs.SetInt ("XPCap", XPCap);
             PlayerPrefs.SetInt("XP", XP);
             PlayerPrefs.SetInt ("XPCapMultiplier", XPCapMulitplier);
 
 
         }
         if (Tin <= 4) {
             CraftTinPanelButtonText.text = "CANT AFFORD";
 
         }
     }
 
     public IEnumerator WaitForView()
     {
         
         CraftSteelButtonText.text = "CRAFTED";
 
         CraftSteelButton.interactable = false;
         yield return new WaitForSeconds (2f);
         PlayerPrefs.SetInt ("Iron", PlayerPrefs.GetInt ("Iron") - 1);
         PlayerPrefs.SetInt ("Coal", PlayerPrefs.GetInt ("Coal") - 1);
         PlayerPrefs.SetInt ("Steel", PlayerPrefs.GetInt ("Steel") + 1);
         CraftSteelOn = false;
         CraftSteelButtonImage.fillAmount = 0;
         CraftSteelButtonImageRight.fillAmount = 0;
         CraftSteelButton.interactable = true;
         CraftSteelButtonText.text = "CRAFT";
     }
 
     public IEnumerator WaitForView2()
     {
 
         CraftElectrumButtonText.text = "CRAFTED";
 
         CraftElectrumButton.interactable = false;
         yield return new WaitForSeconds (2f);
         PlayerPrefs.SetInt ("Silver", PlayerPrefs.GetInt ("Silver") - 1);
         PlayerPrefs.SetInt ("GoldOre", PlayerPrefs.GetInt ("GoldOre") - 1);
         PlayerPrefs.SetInt ("Electrum", PlayerPrefs.GetInt ("Electrum") + 1);
         CraftElectrumOn = false;
         CraftElectrumButtonImage.fillAmount = 0;
         CraftElectrumButtonImageRight.fillAmount = 0;
         CraftElectrumButton.interactable = true;
         CraftElectrumButtonText.text = "CRAFT";
     }
 
     public IEnumerator WaitForView3()
     {
 
         CraftTinPanelButtonText.text = "CRAFTED";
         yield return new WaitForSeconds (1f);
         CraftTinPanelButton.interactable = false;
         PlayerPrefs.SetInt ("Tin", PlayerPrefs.GetInt ("Tin") - 4);
         PlayerPrefs.SetInt ("TinPanel", PlayerPrefs.GetInt ("TinPanel") + 1);
         tinpaneltimer = 120;
         CraftTinPanelOn = false;
         tinpaneltimerOn = false;
         CraftTinPanelButtonImage.fillAmount = 0;
         CraftTinPanelButtonImageRight.fillAmount = 0;
         CraftTinPanelButton.interactable = true;
         CraftTinPanelButtonText.text = "CRAFT";
     }
 
 
 
     void StartC()
     {
         StartCoroutine (WaitForView ());
     }
 
     void StartC2()
     {
         StartCoroutine (WaitForView2 ());
     }
 
     void StartC3()
     {
         StartCoroutine (WaitForView3 ());
     }
 
 }

Ok so the "**" show where the problem is and I know that that line will return the error "Operator "-" cant be used on operants "int" and "timespan" I'm trying to figure out a way to subtract the TimeSpan difference between the (oldDate and the currentDate) and a int. If you now how to do this please leave a comment! Maybe it has something to do with Converting it to an Int but i dont know how to.