C# Masked textbox with date format skips first character

Question

I have a problem in a winform app. I have several masked textbox which use the short date mask (//____), the problem is that if I select all the text (either with Ctrl + a or from code) and I write a new date is like that the first character goes at an 11th position (the mask disappear and i see only the character I wrote) and if I press backspace the text becomes something like this _1/01/1979 and I have to select all again and press backspace or delete everything.

I handled in this way

private void maskedTxt_KeyPress(object sender, KeyPressEventArgs e)
        {
            MaskedTextBox msk = sender as MaskedTextBox;
            if (msk != null)
            {
                if (!string.IsNullOrEmpty(msk.Text.Replace("/", string.Empty).Replace(":", string.Empty).Trim()) && _focused)
                {
                    _focused = false;
                    SendKeys.SendWait("{BACKSPACE}");
                }
            }
        }

_focused is a Boolean variable that I set to true if a validation error happens at the leave event of the masked textbox (invalid date, the date is too big or too small etc...) so that when someone enters the textbox the text can be written correctly

Is there a better way to handle this "error" or this is good? I tried it and it worked but a lot of people will have to use this application and probably there will be some errors along the way. Thanks