This dojo modifies the accepted answer in this stack overflow post.
In the dataBound event, you will want to prepend the marker to your current view, otherwise you will lose it when navigating (unless you don't want it to always be there, perhaps if you only want it on a certain day? Then don't put it in the dataBound and just put it after your scheduler initialization).
In the example, I just set the top to the offsetTop of a time slot, which has limited capabilities because you are limited to whatever your slot time ranges are (You could also set it to the end of a slot).
You will have to calculate the top value if you want it to go between time slots for random times like, say, 9:24 am instead of just 9am or 9:30am. You can't just set it by desired date and time.
Change the border-left-color in the css for new-time-marker to change the markers color. As mentioned in the stack overflow post, the transform is necessary to make sure the center point is on the line you desire to set it at, otherwise the top of the arrow will be on that line.
Increase the size in the border style of the css to increase or decrease the size of the arrow, but make sure the translate is the same as the border size, but negative.