All you really need at this age is a library where you can borrow lots of books and a set of blocks. Talk, sing, read, and count objects (one, two, three blocks!) to your child. It sounds like you have a good daycare, which has a preschool curriculum, so you're good on that front. Don't worry about future schooling until you know where you're moving to with the next deployment.

Start looking around December the year before you want to send your kid. While most preschools (at least the normal ones) are not admissions based, many do have waiting lists and you don't want to be too far down them.

Assuming this is a real post... many daycare centers for young children turn into "preschool" curriculum when the child is 3 or so. If you don't work full time and your children aren't in daycare, parents often put them in some part-time private preschool starting at 3 or sometimes 2. I'm not in the District but there is free preschool for 3 and 4 year olds at local public and charter schools with info here: http://osse.dc.gov/service/early-learning-services-parents-pre-kindergarten-services

I haven't read the original article but I'm absolutely sure your 18 month old is not too late for anything.