Question: In what year did the city with a postcode area identified by DE1 become an official city ?

text: DE postcode area
Coverage
The approximate coverage of the postcode districts :
The DE postcode area, also known as the Derby postcode area, is a group of 23 postcode districts in central England, which are subdivisions of 11 post towns. These postcode districts cover south and central Derbyshire (including Derby, Alfreton, Ashbourne, Bakewell, Belper, Heanor, Ilkeston, Matlock, Ripley and Swadlincote), parts of east Staffordshire (including Burton upon Trent) and north-west Leicestershire, and very small parts of Nottinghamshire.
Postcode district Post town Coverage Local authority area
DE1 DERBY Derby city centre Derby
DE3 DERBY Derby ( Mickleover ) Derby , South Derbyshire
DE4 MATLOCK Sector 2 : Darley Dale , Beeley , Rowsley , Winster , Darley Bridge , Elton , Bonsall Sector 3 : Matlock , Matlock Bath , Cromford Sector 4 : Middleton-by-Wirksworth , Wirksworth , Bolehill Sector 5 : Crich , Holloway , Lea , Tansley Derbyshire Dales , Amber Valley
DE5 RIPLEY Ripley , Codnor , Denby , Waingroves , Butterley , Pentrich Amber Valley
DE6 ASHBOURNE Ashbourne , Hulland Ward , Weston Underwood Derbyshire Dales , Amber Valley , South Derbyshire , Derby , East Staffordshire
DE7 ILKESTON Horsley Woodhouse , Ilkeston , Morley , West Hallam Erewash , Amber Valley , Broxtowe
DE11 SWADLINCOTE Swadlincote , Church Gresley , Woodville , Newhall , Hartshorne , Castle Gresley , Blackfordby , Albert Village South Derbyshire , North West Leicestershire
DE12 SWADLINCOTE Appleby Magna , Donisthorpe , Linton , Measham , Overseal , Rosliston , Walton-on-Trent North West Leicestershire , South Derbyshire
DE13 BURTON-ON-TRENT Alrewas , Barton-under-Needwood , Rolleston on Dove , Tutbury East Staffordshire , Lichfield
DE14 BURTON-ON-TRENT Branston , Burton upon Trent , Newton Solney , Winshill East Staffordshire , South Derbyshire
DE15 BURTON-ON-TRENT Bretby , Stapenhill East Staffordshire , South Derbyshire
DE21 DERBY Derby ( Chaddesden , Oakwood , Spondon ) ; Breadsall , Horsley , Little Eaton Derby , Erewash , Amber Valley
DE22 DERBY Derby ( Allestree , Darley Abbey , Mackworth ( estate ) ) ; Kedleston , Mackworth ( village ) , Quarndon Derby , Amber Valley
DE23 DERBY Derby ( Heatherton Village , Littleover , Normanton , Pear Tree , Sunny Hill ) Derby , South Derbyshire
DE24 DERBY Derby ( Allenton , Alvaston , Boulton , Osmaston , Shelton Lock , Sinfin , Wilmorton ) ; Stenson Fields Derby , South Derbyshire
DE45 BAKEWELL Ashford-in-the-Water , Bakewell , Baslow , Chatsworth , Edensor , Hassop , Monyash , Youlgreave Derbyshire Dales
DE55 ALFRETON Sector 1 : Swanwick Sectors 2 and 3 : Alfreton , South Normanton Sector 4 : Riddings , Greenhill Lane , Leabrooks , Somercotes Sector 5 : Tibshelf , Newton , Blackwell , Hilcote Sector 6 : Wessington , Higham , Stonebroom , Morton , Shirland Sector 7 : Alfreton , South Wingfield Amber Valley , Bolsover
DE56 BELPER Ambergate , Belper , Duffield , Heage , Holbrook , Kilburn , Milford Amber Valley
DE65 DERBY Burnaston , Egginton , Etwall , Findern , Hatton , Hilton , Milton , Repton , Willington South Derbyshire
DE72 DERBY Ambaston , Aston-on-Trent , Borrowash , Breaston , Church Wilne , Draycott , Elvaston , Ockbrook , Risley , Shardlow , Weston-on-Trent Erewash , South Derbyshire , North West Leicestershire