Why
char myData[6];
is better than
char myData[5]; ?
The maximum value that analogRead can return is +1023. That's 5 characters. There also needs to be room in the array for a trailing NULL. That makes 6 characters, so "char myData[6]" allocates room for all 6 characters. Therefore it is better, because there can be no buffer overrun when the buffer is big enough.
With millis() was no problem. But how transform, for example, 6935 mS into 6.9 s? When milliseconds were divided by 1000, the result was 6, not what was wanted.
long then = millis();
//do something
long now = millis();
int secs = (now - then) / 1000;
This will result in secs = 6.
long then = millis();
//do something
long now = millis();
float secs = ((float)now - (float)then) / 1000.0;
This will result in secs = 6.935.
There is no ftoa function, though, so you have to use sprintf to convert floats to strings.