Hi Kelsey,
If it's one Email that you want to set up, you can add wait steps for the Send Email. That would mean Setting up Wait Steps till Dates (15th, 30th of each month) and then adding the flow of Send Email alternatively between the wait steps. The Date token however, uses the attributes related to lead - e.g. Birthday or System Tokens which render the current date and time - thus that would require creation of separate tokens with their definitions as 15/30 Dates and then using the tokens in wait steps. If you're looking to set this up for multiple emails, different campaigns can be requested using required criteria.
Hope this helps!