Calculating time since previous date

How come this works:

...but this does not work:

I'm trying to use an array to calculate the days since my shades have been charged and I've narrowed down my issue to the difference above. Any insight?

Need index on checktime array in the math?

Yep, that was it! Thanks!

It's interesting that it stores the datetime in different formats depending on if it's a single variable or an array.