I think, I did not understand "for this month I want to show 1/31/2018"
Are you saying
If date is from feb 2018, you want to show jan 31 2018
else you want to show exact date.
I will modify above calculation like below
If YEAR(TODAY()) = YEAR([Order Date]) and MONTH(TODAY()) = MONTH([Order Date]) then
DATEADD( 'day', -1, MAKEDATE(YEAR([Order Date]), MONTH([Order Date]),1))
else [Order Date]
Can you have sample workbook?
Try with this Date calculation.
For Last day of last month.