"DeprecationWarning: datetime.datetime.utcnow() is deprecated" — fixing the Python 3.12 warning in AWS Lambda

The moment you bump a Lambda function to the python3.12 runtime, CloudWatch Logs often starts showing warnings like this — once per cold start, or during your test suite against a Python 3.12 environment: DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC). It doesn't crash your function. But it adds noise to every execution log, buries real errors, and points at a method that will eventually be removed from the language. Here's the full picture — including why it often fires from boto3 rather than your own code. Why Python 3.12 deprecated utcnow() datetime.datetime.utcnow() returns a naive datetime — one with no timezone information attached (tzinfo is None). That sounds harmless until you try to compare it with an aware datetime, which most modern libraries return: import datetime naive = datetime.datetime.utcnow() # tzinfo=None aware = datetime.datetime.now(datetime.timezone.utc) # tzinfo=UTC naive < aware # TypeError: can't compare offset-naive and offset-aware datetimes The naive result also serializes to JSON or a database without any indication that it represents UTC, making it indistinguishable from local time for downstream consumers. Python 3.12 added the DeprecationWarning to every utcnow() and utcfromtimestamp() call. The exact removal version hasn't been announced, but the path is clear — Lambda runtimes track CPython releases, and deprecated functions get removed. The fix: one replacement per call site Old call Python 3.9+ replacement Python 3.11+ shorthand datetime.datetime.utcnow() datetime.datetime.now(datetime.timezone.utc) datetime.datetime.now(datetime.UTC) datetime.datetime.utcfromtimestamp(ts) datetime.datetime.fromtimestamp(ts, datetime.timezone.utc) datetime.datetime.fromtimestamp(ts, datetime.UTC) datetime.UTC (the shorthand) was added in Python 3.11. If your package must stay compatible with Python 3.9 or 3.10 for local testing, use datetime.timezone.utc — it works identically everywhere Python 3.2+. Both replacements return an aware datetime with tzinfo=UTC, so they compare correctly with any other aware datetime, serialize unambiguously, and pass ISO 8601 validation. Find every call in your deployment package grep -r "utcnow\(\)\|utcfromtimestamp" . --include="*.py" Run from your Lambda project root (or unzipped deployment package) to surface every call site — in your code and in vendored dependencies. Why the warning fires from boto3 even if you didn't write utcnow() This is the part that surprises most developers. botocore — the low-level AWS SDK library that boto3 wraps — called datetime.utcnow() internally in several places, most visibly in its request-signing code (auth.py). Multiple GitHub issues tracked this across botocore versions (boto/botocore#3038, #3088, #3201, #3374). AWS has shipped patches for these across botocore releases. The fastest fix is to upgrade: pip install --upgrade boto3 botocore Lambda's managed runtime bundles a pinned boto3/botocore. To get the newer version, include boto3 in your deployment package or a Lambda layer — it takes precedence over the bundled copy. If you use AWS Lambda Powertools, the library fixed its own internal utcnow() calls. Upgrade to get the clean version: pip install --upgrade aws-lambda-powertools Tracing which file is generating the warning To get a full traceback pointing to the exact caller, set this Lambda environment variable: PYTHONWARNINGS=error::DeprecationWarning This turns every DeprecationWarning into an exception, so the next cold start logs a complete stack trace identifying the file and line. Remove the variable once you've traced and fixed the source. After the fix: confirm it's gone The cleanest validation is to run your test suite with warnings promoted to errors: python -W error::DeprecationWarning -m pytest A clean run means no remaining utcnow() or utcfromtimestamp() calls in your code or vendored packages. If pytest surfaces new ones from a transitive dependency, open an upstream issue — or vendor a patched copy while the fix propagates. The broader context: python3.12 is the upgrade target Lambda runtimes python3.8, python3.9, and python3.10 are all deprecated and on track for create/update blocks in Q1 2027. The python3.12 runtime is the recommended migration target — and the utcnow() warning is one of the cleanup items that comes with that upgrade, alongside the distutils, imp, and asyncore removals covered in separate guides. Fixing it now is a one-liner per call site and leaves your logs clean for the signals that actually matter. Discovering deprecated runtimes across your whole AWS account? Run a free EOL scan at eolkits.com/scan — paste your Terraform, SAM, CDK, or Serverless config and get a severity-sorted list of every deprecated runtime, AMI, and OS in under 60 seconds.