printf using the %f formatter may fail in certain locales

Hi, thx for providing this project !

There are some printf's in pretty-time.sh (lines 44 & 47), which fail with an invalid number msg when executed in a locale where the "decimal point" isn't actually a ".", but e.g. a "," (like in de_DE as an example).

I was able to fix this by prepending LC_NUMERIC=C to these printf statements, though I'm not exactly sure if this is the best way to do so.