I haven't gotten to that part yet (putting the totals back together again as text), but I appreciate the follow up and will try that.
BTW, I found that your first suggestion...
«TimeSeconds» = (val(array(«Time»,1,":"))*60)+val(array(«Time»,2,":"))
... works quite well.
I was worried about how I'd capture the decimals, but this works. My times are entered as text in a pattern of mm:ss.0, with a colon between "mm" and "ss", and a decimal point between "ss" and "0" (tenths), so the result of the second array includes whatever follows the decimal point because the decimal point is not a separator. I could keep writing, and strip out the decimal point and what follows from the second array, and then add a 3rd array to grab what follows the decimal point. But as is it appears to be working quite cleanly.
For example, the above statement returns a time of 14:12.9 as 852.90 with "TimeSeconds" field property type set to Number (Float).