your approach has two problems:
1. if b is always set to Now, you should do it in the database, set its default value to getdate()
2. you don't know the format like you are saying
3. it is subject to sql injection attack
you should use a parameterized query, for example
sql = "insert into tables(a,b) values(@users,@dates)"
SqlCommand cmd = new SqlCommand(sql,conn);