It is clearly established in dialogue several times (as well as in a flashback sequence) that the character of Megan Dougherty was 15 in 1971. This would have made the character 22 years old -- yet somehow already a fully qualified Ph.D. psychiatrist with a long-established, thriving practice -- when she first appeared in the 1978 Rockford Files episode Black Mirror (1978).
In the scene where Garner and Harrold enjoy morning-after glow in his father's cabin, she wistfully notes, "You hear that one bird, the high one? It goes exactly every 17 seconds." It's a 50 second scene with one irrelevant edit, and the bird sings only 4 times (around her observation), at 14-, 12-, and 10-second intervals.
A flashback scene, explicitly stated to be set during Easter weekend 1971, shows party-goers listening to "(I Know) I'm Losing You" as performed by Rod Stewart. Easter weekend that year was Friday, April 9 through Sunday, April 11; Stewart's recording of "(I Know) I'm Losing You" was not issued in any format until appearing as a track on his album "Every Picture Tells A Story" on May 28, 1971.