In the early anime episodes, Keroro's childhood seems to take place in the early 80s, right after the Gundam boon. That's basically retconned away later though since it doesn't actually make much sense with some other elements of the series - like the aliens actually being much older than humans.
In later anime episodes, Keroro's childhood is generally depicted as similar to middle 70s Japan (so the same would go to Giroro and Dororo) based on various toys he's shown playing with, like thinly disguised versions of Atlanger and Gattai Yamato. Attempting to give actual dates to them doesn't really work though, because the Keronians are actually much older than humans. Keroro is officially listed as 10500 years old, and although that's in Keronian years, rather than Earth ones, he's older than Mois who he met as a small child but was already teenage looking when she arrived on Earth 500 years before the series. So, even if Keronian years are shorter, by the time Keroro was a child Earth should be nowhere near modern age.