TypeError: can't subtract offset-naive and offset-aware datetimes during sam sync --watch

[Leigh-M]

Description:

When running sam sync --watch with a nested stack containing
an ECR-based Lambda function, the sync fails with a datetime
comparison error in SAM's internal code.

SAM CLI Version:
1.149.0

OS:
Linux 6.8.0-87-generic (Ubuntu)

Steps to Reproduce:

1. Create a SAM template with a nested stack
   (AWS::CloudFormation::Stack) that references a local template:
   RecruiticsMLStack:
   Type: AWS::CloudFormation::Stack
   Properties:
   TemplateURL: ./THEMLTemplate.yaml
   Parameters:
   EnvType: !Ref EnvType

2. The nested template contains a Lambda function using an ECR
   image:
   generateRecruiticsFlags:
   Type: AWS::Serverless::Function
   Properties:
   PackageType: Image
   ImageUri: !Sub "${AWS::AccountId}.dkr.ecr.${AWS::Region}.a
   mazonaws.com/my-repo:latest"

3. Run sam sync --watch:
   sam sync --watch --stack-name my-stack --region eu-west-1
   --parameter-overrides EnvType=dev --template THETemplate.yaml

Expected Behavior:

Infra sync completes successfully.

Actual Behavior:

Build succeeds, but infra sync fails with:

Failed to sync infra. Code sync is paused until template/stack
is fixed.
Traceback (most recent call last):
File "samcli/lib/sync/watch_manager.py", line 269, in
_execute_infra_sync
File "samcli/lib/sync/watch_manager.py", line 199, in
_execute_infra_context
File "samcli/lib/sync/infra_sync_executor.py", line 176, in
execute_infra_sync
TypeError: can't subtract offset-naive and offset-aware
datetimes

Additional Context:

• The nested stack deploys successfully via sam deploy
  (non-watch mode)
• SAM shows this warning before the error: The resource
  AWS::Serverless::Function 'generateRecruiticsFlags' has
  specified ECR registry image for ImageUri. It will not be
  built and SAM CLI does not support invoking it locally.
• The error appears to be in SAM's datetime handling when
  comparing stack update times, not in the template itself

Workaround:

Using sam deploy instead of sam sync --watch works correctly (this is a terrible work-flow of course with poor feedback loop experience compared to sam sync --watch)

[vicheey]

Thanks for reporting this!

I was able to reproduce the issue. The root cause is a datetime timezone mismatch in samcli/commands/sync/sync_context.py - when loading timestamps from .aws-sam/sync.toml, datetime.fromisoformat() returns timezone-naive datetimes if the ISO string lacks the +00:00 suffix (e.g., sync_time = "2025-12-03T22:24:14.920432" instead of sync_time = "2025-12-03T22:24:14.920432+00:00"), but the comparison code expects timezone-aware datetimes.

I've reproduced the issue by removing the +00:00 suffix from the file and update the code to trigger the synce and confrim the issue. The fix is to load all timestamps to be timezone-aware UTC, maintaining consistency with how SAM CLI writes them.

I'll have the fix out in a few moment.

In the meantime, make sure your .aws-sam/sync.toml has the +00:00 suffix