You didn't save the output of detectImportOptions so you don't have an import options object at this point.
simply creates the one element in the struct named opts that you passed to readtable but everything else is its default, NOT what the detectImportOptions function would have divined to be the characteristics of the file.
T = readtable(filepath,opts);
first before resorting to anything more exotic; generally the automagic scanning routines are able to infer "who's who in the zoo" without human intervention.
You can use setvartype, yes, if it turns out to be needed as well as setting the specific time format string, but try the above first.