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If you wanted the cumulative disk usage (as your usage of du suggests) of the regular files that are over 60 days old and need only to be portable to GNU and busybox systems, you can do:

(with LS_BLOCK_SIZE=512 BLOCKSIZE=512 POSIXLY_CORRECT=1 to work around the fact that some ls implementations like GNU ls are not POSIX compliant by default).

If you wanted the cumulative disk usage (as your usage of du suggests) of the regular files that are over 60 days old and need only to be portable to GNU and busybox systems (though note that which commands are included in busybox and what feature they support is configurable at build time, so you can never know if what works with one instance of busybox will work with the next), you can do:

(with LS_BLOCK_SIZE=512 BLOCKSIZE=512 POSIXLY_CORRECT=1 to work around the fact that some ls implementations like GNU ls are not POSIX compliant by default. It won't work with busybox ls which doesn't support -q. However since it always renders newline characters in file paths as ? (which is also not POSIX compliant), -q is not needed there).

That reports the total in number of bytes. Hard links are counted only once (like GNU du does; busybox du doesn't do it if the hardlinks are passed as separate arguments as opposed to found in the traversal of a single directory argument). However, like du, it won't detect the cases where some data is shared between non-hardlinked files, like when files have been copied with cp --reflink=always on filesystems like btrfs or when deduplication is performed by some file systems.

That reports the total in number of bytes. Hard links (or other cases such as bind-mounts where there may be several paths to the same file) are counted only once (like GNU du does; busybox du doesn't do it if the hardlinks are passed as separate arguments as opposed to found in the traversal of a single directory argument). However, like du, it won't detect the cases where some data is shared between non-hardlinked files, like when files have been copied with cp --reflink=always on filesystems like btrfs or when deduplication is performed by the file system.

After (here on a GNU system):

Both give me 49152, which is the cumulative disk usage of both a and b but is different from the sum of their size (28 TiB) or the size of their disk usage (49152 x 2).

On the above example, they both give: 30786325577728 (28 TiB).

POSIXly, and assuming all files are on the same file system, you could do:

LC_ALL=C LS_BLOCK_SIZE=512 BLOCKSIZE=512 POSIXLY_CORRECT=1 \
  find . -type f -mtime +59 -exec ls -nisqd {} + | awk '
    !seen[$1]++ {sum += $2}
    END {print sum * 512}'

(with LS_BLOCK_SIZE=512 BLOCKSIZE=512 POSIXLY_CORRECT=1 to work around the fact that some ls implementations like GNU ls are not POSIX compliant by default).

After (here on a GNU system):

All give me 49152, which is the cumulative disk usage of both a and b but is different from the sum of their size (28 TiB) or the size of their disk usage (49152 x 2).

Or POSIXly (here without the restriction about single file system):

LC_ALL=C find . -type f -mtime +59 -exec ls -nqd {} + |
  awk -v sum=0 '{sum += $5}; END {print sum}'

On the above example, they all give: 30786325577728 (28 TiB).