Our PE ratios are calculated from the Reuters data-set using the current price divided by 12 months of earnings.
Different publications choose different 12 month periods to define their EPS figure - whether using the last annual report, the trailing 12 month period or the current fiscal year forecast.
We have standardised all the PE Ratios for comparison purposes. We focus on either: i) TTM Historic: The historical PE based on trailing twelve months earnings (see here for how this calculated), or: ii) Rolling Forecast: A forward PE based on EPS calculated by weighting the next 2 year's consensus estimates. This is known as the 'rolling PE ratio' in some publications, and the weighting means that all companies can be compared like for like regardless of differing year end dates (see here for how this is calculated).
Other sites may choose to simply use last year's earnings or a simple forecast. We do display those number on the stock report but we don't see the benefit of using historic end of year data when it could be a maximum of 15 months out of date, or risk comparing apples with oranges given different company reporting periods.