How can I check if the DateTimes of an item is within the range of two other dates?

Question

I am a new ASP.NET developer and I have 2 inputs in a form as DateTimes; StartDate and EndDate. I want to make sure that both of them are within the StartDate and EndDate of the Parent from another table. The logic should be as following:

1. Check if the StartDate of the items comes before the EndDate.
2. Check if the StartDate & EndDate of the inserted item is within the range of the StartDate and EndDate in the Customers table

So how can I do this logic?

For your information, I have two tables in the database, namely; Customers and Items.Their schema are: Customers Table:

CREATE TABLE [dbo].[Customers](
    [ID] [int] IDENTITY(1,1) NOT NULL,
    [Name] [varchar](50) NULL,
    [Job] [varchar](50) NULL,
    [StartDate] [datetime] NOT NULL,
    [EndDate] [datetime] NOT NULL,
 CONSTRAINT [PK_Customers] PRIMARY KEY CLUSTERED 
(
    [ID] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

Items Table:

CREATE TABLE [dbo].[Items](
    [ID] [int] IDENTITY(1,1) NOT NULL,
    [Name] [varchar](50) NOT NULL,
    [Description] [varchar](50) NULL,
    [StartDate] [datetime] NOT NULL,
    [EndDate] [datetime] NOT NULL,
    [CustomerID] [int] NOT NULL,
 CONSTRAINT [PK_Items] PRIMARY KEY CLUSTERED 
(
    [ID] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

GO

SET ANSI_PADDING OFF
GO

ALTER TABLE [dbo].[Items]  WITH CHECK ADD  CONSTRAINT [FK_Items_Customers] FOREIGN KEY([CustomerID])
REFERENCES [dbo].[Customers] ([ID])
GO

ALTER TABLE [dbo].[Items] CHECK CONSTRAINT [FK_Items_Customers]
GO

My C# Code:

protected void btnSubmit_Click(object sender, EventArgs e)
    {
        DateTime startDate = Convert.ToDateTime(txtStartDate.Text);
        DateTime endDate = Convert.ToDateTime(txtEndDate.Text);
        if (DateTime.Compare(startDate, endDate) <= 0)
        {
            using (TestDBEntities1 TEntities = new TestDBEntities1())
            {
                var newItem = new Item
                {
                    Name = txtName.Text,
                    Description = txtDesc.Text,
                    StartDate = startDate,
                    EndDate = endDate,
                    CustomerID = Convert.ToInt32(ComboBox1.SelectedValue)
                };
                TEntities.Items.AddObject(newItem);
                TEntities.SaveChanges();
            }
            lblMessage.Text = "Successfully Added :)";
        }
        else
        {
            lblMessage.Text = "Error!!!";
        }
    }

NOTE: I am using LINQ for querying the database. I am only missing the part of doing the logic for checking if the StartDate & EndDate of the inserted item is within the Dates in the Customers table.

Explanation:

For example, let us assume we have StartDate#1 and EndDate#1 for an item that will be inserted. In the Customers table, I have StartDate#2 and EndDate#2. The logic should do the following:

1. check if the StartDate#1 comes before EndDate#1
2. Then, check if StartDate#1 and EndDate#1 are falling within the range of StartDate#2 and EndDate#2