There’s a certain science to nostalgia, an inexact math that tells us when the time has come for misty recollection. And to my reckoning, it’s about a decade too early for “The Nanny Reunion: A Nosh to Remember” (7 p.m., Lifetime). Don’t get me wrong. I was a fan of that slapstick comedy, a British farce with a Yiddish accent. But let’s face it: “The Nanny” ran for six years (beginning in 1993) and has been off the air for five years, ending in 1999. And it’s been in repeats for most of that half-decade.

“Nosh” unfolds like a real reunion, with star/producer Fran Drescher hosting a holiday brunch for returning cast members. Everybody except the world-weary butler Niles (Daniel Davis) shows up to eat and narrate the clip reel. A must for fans of the show and Drescher’s curvaceous clothes-horse character. The rest of us will have to wait until genuine nostalgia kicks in.

• TV Land, the network dedicated to nostalgia, celebrates the “100 Most Memorable TV Moments” (9 p.m.) over the next five nights. While enjoyable for it’s own sake, “Most” points out the medium’s ability to elevate the trivial and reduce the profound to the level of a televised event “consumed” in the safety of our armchairs.

For example, tonight’s countdown includes both the “Clarabelle Speaks” episode of “The Howdy Doody Show” and John Dean’s testimony before the Senate Watergate hearings. Two of tonight’s bottom-20 moments (100 to 81) come from reality TV: The moment Sam from “The Apprentice” heard “You’re fired!” is included, along with the “Real World” episode in which the nose-picking Puck got kicked out of the surveillance pod.

I fully expect the top 20 (Friday) will include both the funeral of Chuckles the Clown from “The Mary Tyler Moore Show” and Jack Ruby’s murder of Lee Harvey Oswald in the basement of the Dallas police station. They’re both memorable TV moments, but is it reasonable to treat them as equivalent?

• Here’s a news flash: those who spent their formative years listening to heavy-metal records may not be the most articulate folk. On “My Coolest Years: The Metalheads” (10 p.m., VH1), guitarist Dave Navarro, the members of Lit and New York Mets catcher Mike Piazza recall their high school years as fans of heavy metal music; they recollect about the concerts they attended, the clothes they wore and the classes they skipped to dedicate themselves to AC/DC, Black Sabbath, Judas Priest, Metallica and other bands.

Two episodes of “Coolest” will air every night this week. Each hour focuses on a different teenage sub-cult, including geeks, jocks, hippies, Deadheads and other cliques. While it’s vaguely interesting to note that Juliette Lewis grew up liking rap or that the reactionary shrew Ann Coulter attended a lot of Grateful Dead concerts, hour-long helpings of these “Coolest” memories seem padded and self-indulgent. Just in case you’re wondering, the first installment of the evening is “My Coolest Years: Summer Vacations” (9 p.m., VH1).

Tonight’s other highlights

• From California to Tennessee on “Trading Spouses” (7 p.m., Fox).
• Drew Carey and Roseanne Barr appear at “The New York Comedy Festival” (7 p.m., ABC).
• Ray suffers from mistletoe fever on a vintage episode of “Everybody Loves Raymond” (8 p.m., CBS) from 1997.
• Money laundering on “Las Vegas” (8 p.m., NBC).
• Seattle hosts Dallas on “Monday Night Football” (8 p.m., ABC).
• A lonely woman gets a second shot at happiness in the 2004 holiday fantasy “Eve’s Christmas” (8 p.m., Lifetime).
• A drive-by shooting roils the waters of the rap scene on “CSI: Miami” (9 p.m., CBS).
• Scheduled on “Dateline” (9 p.m., NBC): Princess Diana divulges marital woes on recently excavated secret tapes.

Series notes

Lords of the rings on “Still Standing” (7 p.m., CBS) … Career crises on “One on One” (7 p.m., UPN) … Simon’s temptation on “7th Heaven” (7 p.m., WB).

Teen tantrums on “Listen Up” (7:30 p.m., CBS) … Gifts that keep on giving on “Half & Half” (7:30 p.m., UPN).

Pageant time on “The Swan” (8 p.m., Fox) … Toni puts on a false front on “Girlfriends” (8 p.m., UPN) … Andy keeps a secret on “Everwood” (8 p.m., WB) … Finance trumps feeling on “Two and a Half Men” (8:30 p.m., CBS) … Room for improvement on “Second Time Around” (8:30 p.m., UPN).

Late night

Renee Zellweger and Dr. Chris Raxworthy appear on “Late Show with David Letterman” (10:35 p.m., CBS) … Jay Leno hosts Kevin Spacey and Clay Aiken on “The Tonight Show” (10:35 p.m., NBC) … Michael Moore, Larry the Cable Guy and Crossfade appear on “Jimmy Kimmel Live” (11:05 p.m., ABC).

Alan Cumming, Vivica A. Fox and Ricky Fante chat on “Late Night with Conan O’Brien” (11:35 p.m., NBC) … Jason Alexander hosts Stephen Collins, Rachel Bilson and Jon Brion on “The Late, Late Show” (11:37 p.m., CBS).