There is a bug in
Issuer which causes a subtraction overflow when
rateStalePeriod setting is set greater than the current timestamp. To prevent issues when running tests,
fix the code so that it will allow any timestamp if
rateStalePeriod is greater than current timestamp.
Though this fix has no effect on a live network (unless for some reason rate stale period was set too large by accident), this will make it much easier to test synthetix contracts in a local network setting.
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