emerge --sync: >>> SERVER OUT OF DATE: rsync://mirror...

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Error

Example:

>>> Syncing repository 'gentoo' into '/usr/portage'...
* Using keys from /usr/share/openpgp-keys/gentoo-release.asc
* Refreshing keys via WKD ...                                                                                                                                                         [ ok ]
>>> Starting rsync with rsync://gentoo.mirrors.ovh.net/gentoo-portage...
>>> Checking server timestamp ...
receiving incremental file list
timestamp.chk

Number of files: 1 (reg: 1)
Number of created files: 0
Number of deleted files: 0
Number of regular files transferred: 1
Total file size: 32 bytes
Total transferred file size: 32 bytes
Literal data: 32 bytes
Matched data: 0 bytes
File list size: 41
File list generation time: 0.001 seconds
File list transfer time: 0.000 seconds
Total bytes sent: 104
Total bytes received: 132

sent 104 bytes  received 132 bytes  472.00 bytes/sec
total size is 32  speedup is 0.14

>>>
>>> SERVER OUT OF DATE: rsync://gentoo.mirrors.ovh.net/gentoo-portage
>>>
>>> In order to force sync, remove '/usr/portage/metadata/timestamp.chk'.
>>>

There are two simple fixes:

1) simply wait for an hour or for a day, and see if the problem fixes itself. The issue is related to timestamps, and if the mirror has a timestamp that is older than your local copy, this error might be triggered. After a short while, the mirror will probably get updated, fixing the problem. Just wait and see.

2) Or do what it says: delete the file:
# rm /usr/portage/metadata/timestamp.chk
It will remove the timestamp from your local copy, and thus remove the comparison point. However, if there is really a problem with the mirror, which may have a very outdated copy of the portage tree, you will that old copy. In such a case, it may be better to investigate the reliability of the mirror, and check the actual date of their copy.

If you sync from and out-of-date mirror a second time, you might get an error like this one:

* Manifest is over 24 days old, this is suspicious!
* You may want to try using another mirror and/or reporting this one:
*   rsync://gentoo.mirrors.ovh.net/gentoo-portage
*
* Manifest timestamp: 2020-05-31 22:38:18 UTC
* Valid OpenPGP signature found:
* - primary key: DCD05B71EAB94199527F44ACDB6B8C1F96D8BF6D
* - subkey: E1D6ABB63BFCFB4BA02FDF1CEC590EEAC9189250
* - timestamp: 2020-05-31 22:38:18 UTC

with, in August 2020, a Manifest timestamp dating from May of the same year. Find an up-to-date mirror!

Solution

Use mirrorselect to select a more up to date mirror:
https://wiki.gentoo.org/wiki/Mirrorselect

Use /usr/bin/emerge-webrsync to sync the portage tree.

See also:
https://www.gentoo.org/support/rsync-mirrors/
https://wiki.gentoo.org/wiki/Local_Mirror