Sentence:  One big rule that we never ever break is to open a can of ale before <WSD>beer o'clock</WSD>. Luckily this can be anytime between 1000 and 1300 hours and woe-betide anyone on watch who forgets to waken the other at this crucial time of day. 
Meaning of the word marked with <WSD> tags:  The time of the first beer (or alcoholic beverage) of the day.