Get CreateDate and UpdateDate in mvc consuming a webapi

Question

(Controller)

[ResponseType(typeof(void))]
public IHttpActionResult PutVENDEDOR(int id, VENDEDOR vENDEDOR)
{
if (!ModelState.IsValid)
{
return BadRequest(ModelState);
}
if (id != vENDEDOR.ID_VENDEDOR)
{
return BadRequest();
}
db.Entry(vENDEDOR).State = EntityState.Modified;
try
{
//vENDEDOR.DT_ATUALIZA = DateTime.Now.ToString();
db.SaveChanges();
}
catch (DbUpdateConcurrencyException)
{
if (!VENDEDORExists(id))
{
return NotFound();
}
else
{
throw;
}
}
return StatusCode(HttpStatusCode.NoContent);
}
// POST: api/Vendedor
[ResponseType(typeof(VENDEDOR))]
public IHttpActionResult PostVENDEDOR(VENDEDOR vENDEDOR)
{
if (!ModelState.IsValid)
{
return BadRequest(ModelState);
}
//vENDEDOR.DT_INSERCAO = DateTime.Now.ToString();
db.VENDEDOR.Add(vENDEDOR);
try
{
db.SaveChanges();
}
catch (DbUpdateException)
{
if (VENDEDORExists(vENDEDOR.ID_VENDEDOR))
{
return Conflict();
}
else
{
throw;
}
}
return CreatedAtRoute("DefaultApi", new { id = vENDEDOR.ID_VENDEDOR }, vENDEDOR);
}

(Class)

[Required]
[DataType(DataType.Date)]
[DisplayFormat(DataFormatString = "{0:dd/MM/yyyy HH:mm}", ApplyFormatInEditMode = true)]
public DateTime DT_INSERCAO { get; set; }
[DataType(DataType.Date)]
[DisplayFormat(DataFormatString = "{0:dd/MM/yyyy HH:mm}", ApplyFormatInEditMode = true)]
public DateTime DT_ATUALIZA { get; set; }

The problem is, when i update a register, the insert date becomes null