If you only use time lines, then it may be possible to change the heading on the item lines to something more relevant, like days for example. However, that again would be across all invoices.
[Edit]
For example, you could change the headings, similar to this (or whatever you wish for that matter):
The headings are customisable with a bit of CSS. There’s a guide to this in our knowledge base, but please let me know if you would like help to add this to your account 