Where is this data coming from and how much control do you have over the format of the array, because having the dates as an un-categorized index at the same level with other indexes will make looping over the data slow and as hard as possible, requiring that all the index values be parsed to find which ones are dates/not the other indexes? Also, will the dates be in correct order in the array, since that date format is not directly sortable?
Short-version: store the date arrays together under a main index, such as [dates], and use a YYYY-MM-DD format internally for date values.