try this code
protected void bindtimedropdown()
{
string ktr, mtr;
for (int k = 0; k < DateTime.Now.Hour; k++)
{
for (int m = 0; m < DateTime.Now.Minute; m = m + 30)
{
if (k < 10)
{
ktr = "0" + k.ToString();
}
else
{
ktr = k.ToString();
}
if (m < 10)
{
mtr = "0" + m.ToString();
}
else
{
mtr = m.ToString();
}
DropDownList1.Items.Add(new ListItem(ktr + ":" + mtr));
}
}
}