Recurring dates within a range
TL;DR; skip to if/else if/else statements

I can't say much because there is so little context, it looks like your code functions properly and efficiently,
Your indentation on the other hand is a little weird to me, I would do it like this with out so many tabs and new lines
switch (true) {
    case (interval == 7):
        //weekly
        //Push in the selected dates in the selected array.
        for (var i = 0; i &lt; between.length; i += 7) {
            selected.push(between[i]);
        }
        break;
    case (interval == 30):
        //Monthly
        //Push in the selected dates in the selected array.
        for (var i = 0; i &lt; between.length; i += 30) {
            selected.push(between[i]);
        }
        break;
    case (interval == 15):
        //Bi-Monthly
        //Push in the selected dates in the selected array.
        for (var i = 1; i &lt; between.length; i += 15) {
            selected.push(between[i]);
        }
        break;
    case (interval == 0):
        //One time event
        //Push in the selected dates in the selected array.
        for (var i = 0; i &lt; 1; i++) {
            selected.push(between[i]);
        }
        break;
    default:
        return undefined;
}

EDIT:
Mateon1's answer points out a break; that isn't indented correctly, I have fixed this in my code snippet.

after removing indentation I noticed that your one time event is smelly, you should just select the first date
    case (interval == 0):
        //One time event
        //Push in the selected dates in the selected array.
        selected.push(between[0]);            
        break;

you use more overhead for a for loop than is necessary, don't get into the habit of using bigger things than you need.
After all that I also removed some unneeded comments they were just saying the same thing that the code was telling me outright.
Here is the Code that is left
switch (true) {
    case (interval == 7):
        //weekly
        for (var i = 0; i &lt; between.length; i += 7) {
            selected.push(between[i]);
        }
        break;
    case (interval == 30):
        //Monthly
        for (var i = 0; i &lt; between.length; i += 30) {
            selected.push(between[i]);
        }
        break;
    case (interval == 15):
        //Bi-Monthly
        for (var i = 1; i &lt; between.length; i += 15) {
            selected.push(between[i]);
        }
        break;
    case (interval == 0):
        //One time event
        selected.push(between[0]);            
        break;
    default:
        return undefined;
}


if/else if/else statements
After viewing the other answer, I think that this would be much better written as an if/else if/else statement like the following
if (interval == 7) {
    for (var i = 0; i &lt; between.length; i += 7) {
        selected.push(between[i]);
    }
} else if (interval == 15) {
    for (var i = 0; i &lt; between.length; i += 15) {
        selected.push(between[i]);
    }
} else if (interval == 30) {
    for (var i = 0; i &lt; between.length; i += 30) {
        selected.push(between[i]);
    }
} else if (interval == 0) {
    selected.push(between[0]);
} else {
    return undefined;
}

and since I have gone this far I think that we could actually create a nice little function that does all of this when given an interval, I included the tests to make sure that the interval stays within the constraints but you could alter that to make it much more extendable.
function GetRecurringDates(interval) {
    if (interval != 0 || interval != 7 || interval != 15 || interval != 30) {
        return undefined;
    }
    for (var i = 0; i &lt; between.length; i += interval) {
        selected.push(between[i]);
    }
}

From @Rolfl's answer I noticed that I too missed the 0-base array bug, so here is the function with the fix
function GetRecurringDates(interval) {
    if (interval != 0 || interval != 7 || interval != 15 || interval != 30) {
        return undefined;
    }
    for (var i = 0; i &lt; between.length - 1; i += interval) {
        selected.push(between[i]);
    }
}

I wrote it like that so that the user doesn't have to know that the code is based on a 0-based array.

You might want to look into using JavaScript's Built in Date functionality.