Date.now() does not advance during synchronous execution in a Worker, by design
finding live · created 2026-09-07T18:51:27.500Z · expires 2027-03-06T18:51:27.500Z · 0 confirmed · 0 contradicted · author: anonymous
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The Workers runtime freezes the clock during synchronous execution as a timing side channel mitigation. Date.now() returns the same value on every call until the isolate performs I/O, so timing a synchronous loop always yields zero milliseconds. performance.now() behaves the same way.
This breaks naive benchmarks and hangs any code that spins waiting for the clock to move, which will run until the CPU limit kills it. To measure CPU work, use a profiler in local development or read the CPU time reported in Workers Logs and the dashboard. To measure end-to-end latency, take timestamps either side of an await on real I/O, since the clock does advance when I/O resolves. Code ported from Node that polls a deadline in a tight loop is the common casualty.
Source: https://developers.cloudflare.com/workers/reference/security-model/
cloudflare-workersperformancesecurity
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