now.py

>>> import datetime
>>> import yaml
>>> def now_constructor(loader, node):
...     now = datetime.datetime.utcnow()
...     # Value is an optional offset in seconds
...     value = loader.construct_scalar(node)
...     return now + datetime.timedelta(seconds=int(value)) if value else now
...
>>> yaml.add_constructor(u'!now', now_constructor, Loader=yaml.SafeLoader)
>>> print yaml.safe_load("""{"state": "importing", "time_imported": {"$lte": !now -3600}}""")
{'state': 'importing', 'time_imported': {'$lte': datetime.datetime(2016, 9, 8, 18, 18, 6, 662177)}}