object required run time error '424' - excel

Question

i am getting object required run time error in below code at line , i checked sheet names they are correct but still showing same error Sheet1.Range("A1").Value = Date & " " & Time
Private Sub CommandButton1_Click()
Dim username As String
Dim password As String
username = TextBox1.Text
password = TextBox2.Text
Dim info
info = IsWorkBookOpen("D:\TMS_Project\username-password.xlsx")
If info = False Then
Workbooks.Open ("D:\TMS_Project\username-password.xlsx")
End If
Dim x As Integer
x = 2
Do While Cells(x, 1).Value <> ""
If Cells(x, 1).Value = username And Cells(x, 2).Value = password Then
MsgBox "Welcome!"
Sheet1.Range("A1").Value = Date & " " & Time
Selection.NumberFormat = "m/d/yyyy h:mm AM/PM"
UserForm1.Hide
ActiveWorkbook.Close True
End
Else
x = x + 1
End If
Loop
MsgBox "Please check your username or password!"
ActiveWorkbook.Close True
TextBox1.Text = ""
TextBox2.Text = ""
TextBox1.SetFocus
End Sub

Answer 1

When you use Sheet1.Range("A1").Value, Sheet1 is actually the Worksheet.CodeName property, read here on MSDN.
While I think you meant to use the worksheet, which name is "Sheet1", then you need to use Worksheets("Sheet1").Range("A1").Value.
If you would have defined and set your Worksheet object, you would have been able to track it.
I am using the piece of code below, to verify that no one has changed my sheet's name (or deleted it).
Option Explicit
' list of worksheet names inside Workbook - easy to modify here later
Const ShtName As String = "Sheet1"
'====================================================================
Sub VerifySheetObject()
Dim Sht As Worksheet
On Error Resume Next
Set Sht = ThisWorkbook.Worksheets(ShtName)
On Error GoTo 0
If Sht Is Nothing Then ' in case someone renamed the Sheet (or it doesn't exist)
MsgBox "Sheet has been renamed, it should be " & Chr(34) & ShtName & Chr(34), vbCritical
Exit Sub
End If
' your line here
Sht.Range("A1").Value = Date & " " & Time
End Sub

Answer 2

To use Variables for your Sheets use:
Dim sht as Worksheet
Set sht = Worksheets("Name")
If you are refering a lot to worksheets its a must to use, but also makes it much easier to change later on.