I recently applied for my spouse in F2A category (priority date March, 2018). She in in US on a H1B visa. I am little confused about date of filling application. Is it also applicable for people who is already in US? Thanks for helping me on this !

As others who are currently *legally* staying in the US, your wife could submit Form I-485, to adjust her status, on her PD of "Dates For Filing Applications" (according to her borned country) arrives.