How to make DateTime independent from the current culture?

I cam trying to convert a datetime to string and back, but making it so that it works for all cultures.

I basically have a Textbox (tbDateTime) and a label (lbDateTime). The label tells the user, in which format the software expects the input of tbDateTime. The input of the Textbox will be used for an MySQL command.

Currently it works like this:

lbDateTime.Text = "DD.MM.YYYY hh:mm:ss";   // I live in germany
DateTime date = Convert.ToDateTime(tbDateTime.Text);
String filter = date.ToString("yyyy-MM-dd HH:mm:ss");

Now my question:

• Is it possible to determine the format-string for lbDateTime.Text based on the current culture?
• Which format does the Convert.ToDateTime function uses?

I hope you can help me. I have actually no pc here to test different cultures, so I'm very afraid that I make something wrong.