How do I calculate the Level of the Opposable Thumb Drive?

[Rubix1919]

I know the rules for the Opposable Thumb Drive state "Its level is equal to the last two digits of the year." Does this mean the drive would be, for this year, level 15 or level 6 (adding 1 and 5 together)? Additionally, if it is level 15, shouldn't it be worth more than 1 level and 1 treasure? Or, if it is the last two digits added together, what about much later years (looking at a distant example of 2099 where the level would be 18 (thinking about grand kids and great grand kids here)).

Thank you!