The militia doesn't arrive until ch. 7 ("the recent arrival of a militia regiment in the neighbourhood; it was to remain the whole winter, and Meryton was the headquarters.") In ch. 6, before the milita arrives, it is said that Jane had known Bingley for a "fortnight" so it's been that long since the Meryton Assembly. Andrea is right - if the milita has already been in town, the officers would have been at the assembly and Elizabeth would not have had to sit out a dance and be abused by Darcy.