invalid date error for %m%d%Y format

The date + option sets only the output format, not the input. You'll need to use a different tool or format the string first:

date --date="$(sed 's/^\([0-9][0-9]\)\([0-9][0-9]\)\([0-9][0-9][0-9][0-9]\)$/\3-\1-\2/' <<< '10042015')"

or

date --date="$(sed 's/\(..\)\(..\)\(....\)/\3-\1-\2/' <<< '10042015')"

or simply

date --date="$(sed 's/\(.*\)\(....\)$/\2\1/' <<< '10042015')"

(thanks @Costas). Even though the last two are shorter I would still suggest the first one, because:

1. it's stricter, avoiding strange bugs if handling strings from third party tools or users, and, more importantly,
2. it's more maintainable, since it's more obvious what the code is doing (rearranging three numbers and putting hyphens between them as opposed to moving the last four characters to the start of the string).