If you wish to have Shortened DATE(s) in the custom strings.. you will have to manually edit the XComGame.INT file directly from its default Path location.

(Your Drive:)\Steam\steamapps\common\XCOM 2\XCom2-WarOfTheChosen\XComGame\Localization\INT\XComGame.INT

Relatively simple if you follow these instructions.

1) Search for this header in the file..

[X2StrategyGameRulesetDataStructures]

2) Replace the corresponding default lines (which start on #12107) with those..

m_strMonth0=JAN
m_strMonth1=FEB
m_strMonth2=MAR
m_strMonth3=APR
m_strMonth4=MAY
m_strMonth5=JUN
m_strMonth6=JUL
m_strMonth7=AUG
m_strMonth8=SEP
m_strMonth9=OCT
m_strMonth10=NOV
m_strMonth11=DEC
m_strMonthDayYearLong="<XGParam:StrValue0/!MonthString/>-<XGParam:IntValue0/>"

3) Save the new file. NotePad++ is recommended since it automatically uses the correct (UCS-2 LE BOM) format.

4) Done!

This basically removes the , 2035 year tag from just about anywhere too; Geoscape, Avenger strings at the bottom right UI corner. Small price to pay to have rational text on Posters. ;)