Hank and the Great Glass Elevator - S5-E11
Corrected entry: Hank tells the other guys that his work day begins at 9:00. In the episode "Junkie Business" (Season 2), he told a new employee that Strickland Propane opens at 8:00.
It's Not Easy Being Green - S5-E16
Corrected entry: There is no explanation shown for Bill, Hank, and Dale being able to start Boomhauer's car in the flashback. Boomhauer wouldn't let them drive it, so he obviously didn't give them the keys.
Corrected entry: In seasons 1-4 the Texas-shaped clock in the Hills' kitchen is silver, or gray, seemingly metal. In season 5 and all episodes after, it is just a blank Texas-shaped piece of wood with the wood grain visible, no hands or markings.
Correction: So they replaced the clock with something else.
Correction: So they changed the hours between the episodes. Mr. Strickland is the kind of guy who'd arbitrarily decide one day that 8:00 is too early to start work.
In addition to this, it's also possible that the business opens at 8:00, but Hank does not always arrive until 9:00. I've worked at places where "key-holder" employees would perform some opening or closing duties.