Expiry of passport

Code is working fine but to display the error for 6 months expiry we have to fill all the fields in the form.
so is there any way to show error as soon as the user enter wrong date in field.
 

protected void btnSubmit_Click(object sender, EventArgs e)
    {
        // Calling function
        passportexpiry();
        passport1expiry();
        visaexpiry();

        //Insert into database
        if ((lblError.Text == "") && (lblError2.Text == "") && (lblError3.Text == ""))
        {
            string QueryString = "insert into VisaApp values('" + txtFirstName.Text + "','" + txtMiddleName.Text + "','" + txtLastName.Text + "','" + ddlCurrentNationality.Text + "','" + ddlPreviousNationality.Text + "','" + txtDateOfBirth.Text + "','" + txtPlaceOfBirth.Text + "','" + ddlCountryBirth.SelectedItem.Text + "','" + txtPassportNo.Text + "','" + txtPassportIssueDate.Text + "','" + txtPassportExpiryDate.Text + "','" + txtPassportPlaceofIssue.Text + "','" + ddlPassportCountryIssue.SelectedItem.Text + "','" + ddlCurrentVisaStatus.SelectedItem.Text + "','" + txtVisaExpiryDate.Text + "','" + txtFatherName.Text + "','" + txtMotherName.Text + "','" + ddlMaritalStatus.SelectedItem.Text + "','" + ddlReligion.SelectedValue + "','" + txtEducationalQualification.Text + "','" + txtLanguageSpoken.Text + "','" + txtPermanentNativeAddress.Text + "','" + txtPermanentNativeCity.Text + "','" + txtPermanentNativePOBox.Text + "','" + ddlPermanentCountry.SelectedItem.Text + "','" + txtSponsorName.Text + "','" + ddlSponsorRelationship.Text + "','" + ddlNationality.Text + "','" + txtPassportNumber.Text + "','" + txtPassportDateofIssue.Text + "','" + txtPassportDateofExpiry.Text + "','" + txtVisaFileNumber.Text + "','" + txtSponsorVisaExpiryDate.Text + "','" + txtPhoneNo.Text + "','" + txtAddress.Text + "','" + txtPoBox.Text + "','" + ddlEmirate.Text + "')";
            SqlConnection con = new SqlConnection(cs);
            SqlCommand cmd = new SqlCommand(QueryString, con);
            con.Open();
            cmd.ExecuteNonQuery();
            //Response.Write("Data Inserted");
            Response.Write("<script>alert('Form has been successfully Submtted')</script>");
            con.Close();
        }
    }


        // Validation for passport expiry(6 months)
    private void passportexpiry()
    {
        var value = DateTime.Now;
        // var val = DateTime.Parse(value);
        var val = DateTime.Parse(txtPassportExpiryDate.Text);

        var diffindays = val - value;

        if (diffindays.Days >= (6 * 31))
        {
            lblError.Text = "";
        }

        else
        {
            lblError.Text = "Must have atleast 6 months validity";
        }

    }

    // Validation for passport expiry(6 months)
    private void passport1expiry()
    {
        var value = DateTime.Now;
        // var val = DateTime.Parse(value);
        var val = DateTime.Parse(txtPassportDateofExpiry.Text);

        var diffindays = val - value;

        if (diffindays.Days >= (6 * 31))
        {        
            lblError2.Text = ""; 
        }

        else
        {         
            lblError2.Text = "Must have atleast 6 months validity";
        }
    }

    // Validation for visa expiry(6 months)
    private void visaexpiry()
    {
        var value = DateTime.Now;
        // var val = DateTime.Parse(value);
        var val = DateTime.Parse(txtSponsorVisaExpiryDate.Text);

        var diffindays = val - value;

        if (diffindays.Days >= (6 * 31))
        {
            lblError3.Text = "";
        }

        else
        {
            lblError3.Text = "Must have atleast 6 months validity";
        }
    }

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