Very very simple AFT need help

Hello for the first time! :)
I'm not very good at code, but was trying to figure out how to plot a ATR based line below price chart.

I'm pretty proud that I've created something like this:

Code:

---

ATR_Multiple = Param("ATR Multiple", 2.5, 1, 4, 0.5 );
ATR_Period = Param("ATR Period", 10, 3, 20, 1 );

CalculateATR = C - ATR_Multiple * ATR( ATR_Period );

Plot( CalculateATR,"ATR STOP", colorRed );

---

Now what I would like to change is:
1. add a date selection from which the line will be drawn. I've found sth like this:

Code:

---

Start = Cross( DateNum(), ParamDate("Start date", "2016-09-09" ) );

---

but have no skills to make it work :(

2. Instead of trail-stop line above, I would like it to be drawn horizontally, from a specific date. So basically I'd want to: select a date and plot automatic horizontal line which will work as a permanent Stop Loss Signal from that moment.

3. The last thing, I'm here not just to receive a solution, but to learn how to code, so any additional explanations and comments would be more than welcome :)

Thank you in advance!
Nigel

Very very simple AFT need help