A "sliding time-line" is a phenomenon that often occurs in long-running serialized fiction. It typically involves how characters are depicted as aging, as well as their ties to real-world events and phenomena. It can often result in difficult-to-reconcile continuity errors, or prompt a creative team to engage in significant retcons.
Some long-running comic strips attempt to depict the characters aging realistically, relative to the way time actually passes. Examples of this would be Walt Wallet in Gasoline Alley, or the characters in Doonesbury or For Better or For Worse. Other strips avoid this and show the characters aging very slowly (if at all). For example, the characters in Peanuts remained children throughout the passing decades (though characters like Linus and Sally were shown aging quickly to bring them closer in age to the more established characters).
Dick Tracy has shown an inconsistent approach to aging. For example, in the storyline that introduced the Mole, Steve the Tramp was said to have been released following a seven-year term in prison. However, the characters (most noticeably Junior) had clearly not aged seven years. Similarly, Sparkle Plenty's birthday (and consequently her age) was acknowledged in the strip for several years, though continuing to do so would place Sparkle well into her 60s during a modern-day storyline.
Characters have also been aged and de-aged, depending on the inclination of the creative team. In the 1970s, the Mole was depicted as elderly, white-haired and hard of hearing. He has since appeared more youthful and vigorous. Dick and Tess Tracy's children have gone back and forth in age, with Bonnie going from adult to teenager back to adult and Joe going from pre-teen to teenager back to pre-teen.
Another instance of the sliding time relates to how characters are affected by real-world events. In the early 1940s, Dick Tracy encountered several criminals whose activities were closely tied to the second World War (such as B-B Eyes, Pruneface, and the Brow). This would make Tracy a senior citizen in the modern day, which is clearly not the case (though Tracy did celebrate his 50th birthday in 1981).
A sliding time-line can make continuity difficult to reconcile, though it does not necessarily impact a reader's ability to enjoy the strip on its merits.