I don’t think this is true. Every bank and exchange has a business continuity plan to allow processes to be completed manually during an outage and I’ve never seen a BCP that required any sort of GPS or celestial navigation access in order to correctly time stamp a transaction.

Think about how credit cards used to be “swiped”[1] it was a literal carbon copy of the card. These machines are still distributed and occasionally still used when a network is down. The exact time of the transaction isn’t necessary because the cardholder or account agreement contains language to address overdrafts in these situations.

[1]https://www.mobiletransaction.org/how-to-accept-credit-card-...