Restart Bill Number every day.....

If you wish to update the TextBox as soon as the day changes, do something like this.

using System.Windows.Threading;//reference WindowsBase
//Inside the class declaration
private readonly DispatcherTimer timer = new DispatcherTimer();
private int lastDay;
private void timer_Tick(object sender, EventArgs e)
 {
   if (DateTime.Now.Date.Day!=this.lastDay)
     {
      this.lastDay = DateTime.Now.Date.Day;
      setBillTextBoxToOne() ;//Your update TextBox method
     }
 }

//in the class constructor
 this.timer.Interval = TimeSpan.FromSeconds(15);
 this.timer.Tick += this.timer_Tick;
 this.lastDay = DateTime.Now.Date.Day;
 this.timer.Start();

If the bill number is only needed for reporting or receipt printing and bill information isn't deleted then I wouldn't bother storing it.

If you derive then it you don't run the risk of accidentally writing the same number twice (or more times) to the DB nor do you have to reset a pre-assigned number if a bill doesn't get created when, for example, a customer complains and the manager writes off the cost by way of compensation.

A notional table...

create table bill
(
  issueDate datetime not null 
  constraint df_bill_issued default getdate(),
  -- A computed column that groups all bills by issue date.
  issueDay as floor(cast(issueDate as float)),
  totalNoTax decimal(5,2) not null,
  taxRate decimal(3,2) not null
  constraint df_bill_tax default(0)
)

Set up some noddy data...

insert into bill(issueDate, totalNoTax) values (getdate(), 0.75)
insert into bill(issueDate, totalNoTax) values (dateadd(hh, 1, getdate()), 1.50)
insert into bill(issueDate, totalNoTax) values (dateadd(hh, 2, getdate()), 2.50)
insert into bill(issueDate, totalNoTax) values (dateadd(hh, 3, getdate()), 3.50)
insert into bill(issueDate, totalNoTax) values (dateadd(hh, 4, getdate()), 4.50)

Write a query to get the bill information with a derived bill number..

set ANSI_WARNINGS OFF  -- Squelch null suppression warning.
select 
  count(counter.issueDay)+1 as billNumber,
  bill.issueDate,
  bill.totalNoTax,
  bill.taxRate
from bill
left join (select issueDay, issueDate from bill) as counter
      on counter.issueDay= bill.Issueday
      and counter.issueDate < bill.IssueDate
group by 
  bill.issueDate,
  bill.totalNoTax,
  bill.taxRate

-- billNumber  issueDate               totalNoTax
-- ----------- ----------------------- ---------------------------------------
-- 1           2015-09-18 20:56:38.563 0.75
-- 2           2015-09-18 21:56:38.580 1.50
-- 3           2015-09-18 22:56:38.583 2.50
-- 4           2015-09-18 23:56:38.583 3.50
-- 1           2015-09-19 00:56:38.587 4.50

If you are going to be doing a lot of work where you need repeated access to bill number then consider converting the query to a view...

create view indexedBill 
as
 
select 
  count(counter.issueDay)+1 as billNumber,
  bill.issueDate,
  bill.totalNoTax,
  bill.taxRate
from bill
left join (select issueDay, issueDate from bill) as counter
      on counter.issueDay= bill.Issueday
      and counter.issueDate < bill.IssueDate
group by 
  bill.issueDate,
  bill.totalNoTax,
  bill.taxRate

...then pretend you've got a new table. For example...

select * from indexedBill where billNumber = 1

-- billNumber  issueDate               totalNoTax                              
-- 1           2015-09-18 21:02:07.037 0.75      
-- 1           2015-09-19 00:02:07.040 3.50      

select * from indexedBill where issueDate < '18 september 2015 23:59:00'

-- billNumber  issueDate               totalNoTax                              
-- 1           2015-09-18 21:02:07.037 0.75           
-- 2           2015-09-18 22:02:07.040 1.50           
-- 3           2015-09-18 23:02:07.040 2.50           

And if you really must display a bill number when creating a new bill (why?) then a query along the following lines ought to be adequate if there isn't much concurrent bill creation.

select max(billNumber) + 1 from indexedBill 
where issueDate = floor(cast(getdate() as float))

Based on this[^] i'd strongly recommend to create custom class collection. Then you'll be able to filter collection on SellDate to get max[^] of BillNo via using Linq[^] query.

Steps to do:

1. Create new Console project[^]
2. Add new Class file (CTRL+SHIFT+A)
3. Copy and paste below code
   
   
   VB

   'Bill class to store single Bill object
   Public Class Bill
   	Private aSellDate As DateTime
   	Private aBillNo As Integer = 0
   
   	Public Sub New(_SellDate As DateTime)
   		aSellDate = _SellDate
   	End Sub
   
   	Public Sub New(_SellDate As DateTime, _BillNo As Integer)
   		aSellDate = _SellDate
   		aBillNo = _BillNo
   	End Sub
   	
   	Public Property SellDate As Datetime
   		Get
   			Return aSellDate
   		End Get
   		Set (value As DateTime)
   			aSellDate = value
   		End Set
   	End Property
   	
   	Public Property BillNo As Integer
   		Get
   			Return aBillNo
   		End Get
   		Set(value As Integer)
   			aBillNo = value
   		End Set
   	End Property
   	
   End Class

   
   
4. Add another Class file (CTRL+SHIFT+A)
5. Copy and paste below code
   
   
   VB

   'Bills class is a collection of Bills
   Public Class Bills
       Inherits CollectionBase
   
       'default constructor
       Public Sub New()
           '
       End Sub
   
           'TO DO:
           'Add Item method
           'Add Remove method
           'etc.
   
       Public Function Add(b As Bill) As Bill
           'List is embeded object, it's a generic List 
           'get index of newly added Bill object
           Dim index As Integer = List.Add(b)
           'change BillNo from default (0) to value corresponding to the current day
           'use Linq query
           'NOTE: we need to cast List to specific type via using <code>Cast(Of Type</code>) or <code>OfType(Type)</code> method
           List(index).BillNo = List.Cast(of Bill).Where(Function(a) a.SellDate = b.SellDate).Max(Function(a) a.BillNo) + 1
           Return CType(List(index), Bill)
       End Function
   
   End Class

6. Usage: (Main method)
   
   
   VB

   Sub Main()
   
      'declare variables
       Dim firstDay As Integer = 18
       Dim lastDay As Integer = 20
       Dim colBills As Bills
       Dim b As Bill
   
       'create collection of Bills
       colBills = new Bills()
   
       'add 15 Bills between 18. and 20. September 2015
       For i As Integer = 0 To 14
           Dim randomDay As Integer = CInt(Math.Floor((lastDay - firstDay + 1) * Microsoft.VisualBasic.Rnd())) + firstDay
           b = new Bill(new DateTime(2015,9,randomDay))
           colBills.Add(b)
       Next i
   
       Dim orderedList = colBills.Cast(of Bill).OrderBy(Function(a) a.SellDate).ThenBy(Function(a) a.BillNo)
       Console.WriteLine("Sell date {0} Bill number", "                   - ")
       For Each a As Bill in orderedList
           Console.WriteLine("{0} - {1}", a.SellDate, a.BillNo)
       Next
   
   
   End Sub

Result:

Sell date           - Bill number
2015-09-18 00:00:00 - 1
2015-09-18 00:00:00 - 2
2015-09-18 00:00:00 - 3
2015-09-18 00:00:00 - 4
2015-09-19 00:00:00 - 1
2015-09-19 00:00:00 - 2
2015-09-19 00:00:00 - 3
2015-09-20 00:00:00 - 1
2015-09-20 00:00:00 - 2
2015-09-20 00:00:00 - 3
2015-09-20 00:00:00 - 4
2015-09-20 00:00:00 - 5
2015-09-20 00:00:00 - 6
2015-09-20 00:00:00 - 7
2015-09-20 00:00:00 - 8

For further information, please see:
Creating Classes in Visual Basic .NET[^]
Walkthrough: Defining Classes (Visual Basic)[^]
Enumerable.Cast(Of TResult) Method (IEnumerable)[^]
Enumerable.OfType(Of TResult) Method (IEnumerable)[^]
Introduction to LINQ in Visual Basic[^]
Writing Your First LINQ Query (Visual Basic)[^]
LINQ - Sample Queries[^]