Use non-cached form [!...!].

[[DateTime? &format=`%B %d, %Y` ×tamp=`[+pub_date+]`]]
[[DateTime? &format=`%B %d, %Y` ×tamp=`[+pub_date+]`]]
[[DateTime? &format=`%B %d, %Y` ×tamp=`[*pub_date*]`]]
[[DateTime? &format=`%B %d, %Y` ×tamp=`[+PUBDATE+]`]]
That depends on how FeedX is giving you the value in the [+pubdate+] placeholder. What does it output when you just include the placeholder on itself?