You'll need to ensure that at least one (maybe more) of your devices (perhaps your two core routers would make sense) can access the Internet using UDP port 123.
Then configure ntp server x.x.x.x on them to get them to sync up - use google to find an appropriate time source, or perhaps your ISP will have one...
You can configure multiple NTP sources per device, so you could have your core sync'd up to a couple of servers (it will determine the most accurate time from them) then configure your other internal network devices (routers etc) to sync up to both the core devices.
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