Get time of the previous and next keyframe

You always need to take care to understand what you're looping through and deal with the correct range.

Generally speaking, if you're accessing something with square brackets [x] it will be a 0 based index, while if you're acessing it using round brackets (x) it will be a 1 based index.

The difference being myArray[x] is an 'array' of values and arrays always start from 0, while keyTime(x) is a 'method' and in AE scripting an index for a method (layers, keyframes, etc) will generally start from 1.

If you have say 3 values in an array you'd loop with something like:

for (x = 0; x < myArray.length; x++) {

while with 3 items in a 1 based index it would be:

for (x = 1; x <= myProp.numItems; x++) {

Notice the <= in the second loop. In the first you loop from 0 to 2 (less than length of 3) while in the second it's from 1-3 (less than or equal to 3)

Here's something that covers the full range:

var theProp = app.project.activeItem.selectedProperties[0];

if (theProp.numKeys == 0) alert("no keyframes");
else if (theProp.numKeys == 1) alert("only one key at " + theProp.keyTime(1));
else {

	for (var x = 1; x <= theProp.numKeys; x++) {
		
		if (x == 1) { // first loop, there will be no previous key but there must be a next key as we know there are more than one to be here
			alert("loop " + x + " current= " + theProp.keyTime(x) + " next = " + theProp.keyTime(x+1));
		} else if (x == theProp.numKeys) { // last loop. there will be no next key
			alert("loop " + x + " last= " + theProp.keyTime(x-1) + " current = " + theProp.keyTime(x));
		} else { // there must be both previous and next keys
			alert("loop " + x + " last= " + theProp.keyTime(x-1) + " current = " + theProp.keyTime(x) + " next= " + theProp.keyTime(x+1));
		}

	}
}