Updated blocktrans strings for i18n

One case in particular:
Due <span> [date] (in [time]) </span>
Was not going to be possible to remove from the translator's view.  In place of that removal we added a helpful comment to translators.
The second sentence is wildly confusing. Instead I'd just use an example. 

`{# Translators: Please do not change the span tags. This will end up looking like: "available blah (in UTC)" }`