Use utcfromtimestamp instead of fromtimestamp when setting creation date.

[shubham-padia]

The creation date of a job dag is set from the unix timestamp of the creation date of the corresponding job file. datetime.fromtimestamp() converts the unix timestamp to local time, this may lead to a delay in running the dag if the system running airflow is not using UTC timezone.

Solution:

• datetime.utcfromtimestamp() gives the UTC time based on the unix timestamp.

Testing:

• @michael-kotliar I have tested this maually using cwl-airflow demo --auto, I could not find instructions on how to run the conformance tests mentioned in some of the issues.

[michael-kotliar]

@shubham-padia Thanks for your pull request. The conformance tests are usually run here