Custom column using text mode to pull a specific date out of project schedules - can't sort on this column to get dates in chronological order. Why not??

Hi All! We created a custom column with the below text mode... intent was to pull the "Launch" date from each of our projects so we had it in a report. But now in the report we aren't able to sort that column so dates appear in order of launch date. Anyone experience this before/any resolutions to this?

displayname=Launch Date

listdelimiter=<div>

listmethod=nested(tasks).lists

textmode=true

type=iterate

valueexpression=IF(CONTAINS("LAUNCH",{name}),{plannedCompletionDate})

valueformat=HTML

Thank you for your help!