In the United Kingdom, white Christmases were more common from the 1550s to the 1850s, during the Little Ice Age; the last frost fair on the River Thames, however, was in the winter of 1813–14. The shift from the Julian to the Gregorian calendar in 1752 also slightly reduced the chance of a white Christmas, by moving Christmas earlier in the winter. Before 2006 for betting purposes, a Met Office employee was required to record if any snow fell on the London Weather Centre over the 24 hours of Christmas Day; after 2006 computers were used. An "official" white Christmas is defined by the Met Office as "one snowflake to be observed falling in the 24 hours of 25 December somewhere in the UK", but formerly the snow had to be observed at the Met Office building in London. By the newer measure, over half of all years have white Christmases, with snow being observed 38 times in the 54 years A more "traditional" idea of snow-covered ground is less common, however, with only 4 occasions in the 51 years reporting snow on the ground at 9am at more than 40% of weather stations.