Unlike DCF77, bit 19 is not used for leap second warnings, but is always zero. Instead, bit 1 is used to warn of a positive leap second at the end of the current hour, and bit 2 is used to warn of a (very unlikely) negative leap second. In case of a leap second, an additional zero bit is inserted between bits 2 and 3. This is supposed to be inserted at 23:59:03, during minute 59 of the hour (during which the timestamp for minute :00 is transmitted), so that the minute markers are all broadcast at the correct times, but for the leap second at the end of December 2016, it was apparently inserted at 23:58:03.