lib: timeutil: fix conversion drift

Fix conversion drifts for large deltas by only applying float
operations when the skew requires it. This helps because not all
integers are representable as floats, so large integers are
neccessarily quantised when performing float operations.

When required, floating-point operations are now performed on doubles
instead of floats.

Fixes #37263.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>