"sada" does seem to mean "now" in Serbo-Croatian:
http://www.websters-online-dictionary.org/translation/sadabut not in other Slavic languages, and it would surprise me if I had to look at the female name form to find root words.
With the male form Mirsad on the other hand we get "sad" which means "garden, plantation" in pretty much every Slavic language:
http://www.websters-online-dictionary.org/translation/sadand therefore could be a very old word, used as a root word for a name.
I could not confirm this alternative theory, however.
Rene www.AboutNames.ch