Yes, you can't do that in Engagements.
Engagement Streams have a regular cadence like once a week or every 25th of the month. You can't make it variable.
I recommend setting up two different Streams OR using a regular program if it is important to you that the timing be precise as well as variable. A campaign with a wait step will work very well for this.
See my presentation on this for more:
http://www.slideshare.net/jdavidhill/marketo-engagements-and-drip-nurturing-boston-user-group-dec-10-2013