FlexGridをExcelとしてメモリに保存し、C1Excelを用いてそのセルの書式を明示的に指定したのち、Excelファイルに保存します。　

◎サンプルコード（VB）

Imports C1.Win.C1FlexGrid
Imports System.IO

Public Class Form1
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        Dim col = Me.C1FlexGrid1.Cols(1)
        col.Name = "UPDATEDATE"
        col.Caption = "UPDATEDATE"
        col.Format = "yyyy/MM/dd HH:mm:ss.000"
        Me.C1FlexGrid1(1, 1) = Date.Now

        Me.C1FlexGrid1.AutoSizeCols()
    End Sub

    Private Sub ToolStripButton1_Click(sender As Object, e As EventArgs) Handles ToolStripButton1.Click
        Dim fileName = "test.xlsx"
        Using mem = New MemoryStream()
            Me.C1FlexGrid1.SaveExcel(mem, "test", FileFlags.IncludeFixedCells)
            Dim workbook = New C1.C1Excel.C1XLBook()
            workbook.Load(mem)
            Dim rows = workbook.Sheets("test").Rows
            For row = 1 To rows.Count - 1
                workbook.Sheets("test")(row, 1).Style.Format = "yyyy/MM/dd HH:mm:ss.000"
            Next
            workbook.Save(fileName)
        End Using

        Process.Start(fileName)
    End Sub
End Class

◎サンプルコード（C#）

using C1.C1Excel;
using C1.Win.C1FlexGrid;

namespace prj_C1FlexGrid
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            Column col = this.c1FlexGrid1.Cols[1];
            col.Name = "UPDATEDATE";
            col.Caption = "UPDATEDATE";
            col.Format = "yyyy/MM/dd HH:mm:ss.000";
            this.c1FlexGrid1[1, 1] = DateTime.Now;

            this.c1FlexGrid1.AutoSizeCols();
        }

        private void toolStripButton1_Click(object sender, EventArgs e)
        {
            string fileName = "test.xlsx";

            using (var mem = new MemoryStream())
            {
                this.c1FlexGrid1.SaveExcel(mem, "test", FileFlags.IncludeFixedCells);
                mem.Position = 0;  
                var workbook = new C1XLBook();
                workbook.Load(mem);
                var sheet = workbook.Sheets["test"];
                for (int row = 1; row < sheet.Rows.Count; row++) 
                {
                    sheet[row, 1].Style.Format = "yyyy/MM/dd HH:mm:ss.000";
                }
                workbook.Save(fileName);
            }

            Process.Start(new ProcessStartInfo(fileName) { UseShellExecute = true });
        }
    }
}