Article-Level Metrics provide new ways to look at the impact of scholarly research. Two important concepts are a) to track metrics for individual scholarly articles instead of using numbers aggregated by journal, and b) to go beyond citations and also include usage stats and altmetrics.
Article-Level Metrics is also doing something else: instead of tracking impact by year, it looks at usage, altmetrics and citations in real-time. There might have been technical reasons to do so 20 years ago, but there really is no longer any reason why scholarly impact should be tracked on a yearly basis in 2013. Unfortunately there is one big stumbling block:
The publication date of a scholarly article is often difficult or impossible to obtain. Publication year may be the only available information.
A good example is CrossRef. They provide a lot of interesting metadata about an article and make this information available in a very nice search interface. But they only require the publisher to provide the publication year, information about the publication month and day is optional. There are many other examples of journals and services that just can’t tell you when exactly an article was published. This might have made sense when periodicals were printed on paper, but doesn’t work for digital content.