From the time-to-seconds doc-string:
"Convert time value TIME to a floating point number.  You can use
`float-time' instead."
Looking at both functions, there doesn't seem to be any difference other
than float-time being implemented in C.  Should time-to-seconds just be
aliased to float-time?

Patch to time-date.el:
< ;;;###autoload
< (defun time-to-seconds (time)
<   "Convert time value TIME to a floating point number.
< You can use `float-time' instead."
<   (with-decoded-time-value ((high low micro time))
<     (+ (* 1.0 high 65536)
<        low
<        (/ micro 1000000.0))))
---
> (defalias 'time-to-seconds 'float-time)

Thanks,
Nathaniel Flath