How to convert the following columns from decimal to datetime

i have to convert the following columns from decimal to datetime. please advice a clear syntax

[ExpDate] [decimal](18, 0) NULL yymmdd   to    ExpirationDate datetime(6) 
[DateLastPlastic] [decimal](18, 0) NULL, ymmdd  to   PrevPlasticDate datetime(6) 
[DateLastPinMailer] [decimal](18, 0) NULL, ymmdd  to   PrevPinMailerDate datetime(6)