dual dating
...that due to the change from the Julian to the Gregorian calendar in 1582, English letter writers often used two dates on their letters, a practice known as dual dating?
...that due to the change from the <a href="./Julian_calendar" id="mwFPc" rel="mw:WikiLink" title="Julian calendar">Julian</a> to the <a href="./Gregorian_calendar" id="mwFPg" rel="mw:WikiLink" title="Gregorian calendar">Gregorian calendar</a> in 1582, English letter writers often used two dates on their letters, a practice known as <b id="mwFPk"><a href="./Dual_dating" id="mwFPo" rel="mw:WikiLink" title="Dual dating">dual dating</a></b>?