An old joke is that in January (the first month of the year), a check writer mistakenly writes the old year on a check. That joke has been cited in print since at least 1969.
 
Late night television comedian Johnny Carson (1925-2005) told the joke about the Chinese new year. (“It’s the Year of the Rabbit, and I’m still writing Year of the Ox on my checks!”) Late night television comedian David Letterman has told the joke about Rosh Hashanah, the Jewish new year. (“It’s Rosh Hashanah, the Jewish new year 5768, and I’m still writing 5767 on my checks!”) The Rosh Hashanah joke has been popular since at least the 1990s.
 
A joke in The New Yorker (January 14, 2008) showed two men living in the time of Christ, with one saying:
 
“There I go—still writing ‘B.C.’ on my checks.”
 
 
Wikipedia: Rosh Hashanah
Rosh Hashanah (Hebrew: ראש השנה‎, literally “head [of] the year”) is the Jewish New Year. The Biblical name for this holiday is called Yom Teruah (Hebrew: יום תרועה‎, literally “day [of] shouting/raising a noise”) or the Feast of Trumpets according to the correct biblical calendar of the 1st and 2nd temple period, not Rosh Hashanah. It is the first of the High Holy Days or Yamim Nora’im (“Days of Awe”) which usually occur in the early autumn of the Northern Hemisphere. Rosh Hashanah is a two-day celebration, which begins on the first day of Tishrei. The day is believed to be the anniversary of the creation of Adam and Eve, the first man and woman, and their first actions toward the realization of humanity’s role in God’s world. Rosh Hashanah customs include sounding the shofar (a hollowed-out ram’s horn) and eating symbolic foods such as apples dipped in honey to evoke a “sweet new year”.
 
Wikipedia: Hebrew calendar
The Hebrew or Jewish calendar (הַלּוּחַ הָעִבְרִי, ha’luach ha’ivri) is a lunisolar calendar used today predominantly for Jewish religious observances. It determines the dates for Jewish holidays and the appropriate public reading of Torah portions, yahrzeits (dates to commemorate the death of a relative), and daily Psalm readings, among many ceremonial uses. In Israel, it is used for religious purposes, provides a time frame for agriculture and is an official calendar for civil purposes, although the latter usage has been steadily declining in favor of the Gregorian calendar.
       
8 January 1969, Boston (MA) Herald Traveler, “Hub-Bub” by Jim Morse, pg. 15, col. 1:
I’m still writing 1968 on my checks. I suppose that’s because I’m still paying bills for 1967.
 
19 January 1990, The New Mexican (Santa Fe, NM), “Making those 90-something predictions” by Anna Quindlen, pg. D-1, col. 1:
Two weeks into the ‘90s, and I’m still writing 80-something on my checks.
 
Google Groups: alt.non.sequitur
The Comeuppance of Emily
Steve Connelly
2/17/94
(...)
So, here it is already the Year of the Dog, and I’m still writing Year of the Woman on my checks!
     
Google Groups: bit.listserv.wpwin-l
Running DOS programs from WPWIN
Chris!
12/28/95
(...)
5756?!?!? I’m still writing 5755 on my checks!
 
Google Groups: rec.arts.tv.mst3k.misc
HEY GUYS!!!!!!!!!!!!!!! GUESS WHAT????????!!!!!!!!!!!!!!!
Josh Forman
11/18/96
(...)
It’s the Jewish New Year and I’m still writing 5756 on my checks.
 
Google Groups: alt.fan.letterman
Old jokes, revisted
Bill Spencer
2/18/99
In article <19990218075639.05472.00002704@ng-ce1.aol.com>, Cantucan

wrote:
> The joke Dave made about still writing Year of the Pig instead of Year of the
> Rabbit on his checks was used by Johnny Carson in the early eighties.  I
> remember it well.  I think one of Dave’s staff writers must have seen the
> Carson version and subconscientiously tucked it away for future reference.
 
Dave tells a version of that joke every year at this time.  Carson did too, but it still gets a laugh.
 
Google Groups: alt.fan.letterman
Funniest running Letterman bit ?
Rick Diamant
6/19/00
I just remembered another annual bit - every Rosh Hashana, when Dave wishes his Jewish viewers and Paul a happy New Year.  He always says I’m still writing 5959 (or whatever the previous Jewish year was) on his checks.  I know it’s coming, but it still cracks me up.
 
Google Groups: alt.fan.letterman
Fri. 9/26 Big Show Synopsis
Brady
9/27/03
Hey, look—it’s a bona fide, full-fledged ‘Big Show Synopsis.’ Here’s how I saw it ...
Monologue ... Highlights: “Thank you ladies and gentlemen. Welcome to the show. Before we begin ... Happy Rosh Hashanah. Happy Rosh Hashanah to everybody. If you’re not aware of this, it’s the Jewish New Year—5764. 5764 ... Here’s the funny thing: I’m still writing 5763 on my checks.” (Paul: “Every year ... Every year you do that joke!”)
 
Condé Nast Store
“There I go—still writing ‘B.C.’ on my checks.” - New Yorker Cartoon
By: Paul Noth Item #: 8546253
Published January 14, 2008
“There I go—still writing ‘B.C.’ on my checks.”
Two ancient Greco-Roman men at a desk.
   
Martin Rottler blog
Raw DS Article: Ruminations on a New Year
SEPTEMBER 28, 2008
To the untrained observer, wishing someone a “Happy New Year” at the end of September might seem a bit out of the ordinary. Even stranger to that person would be the fact that around the world an entire group of people is welcoming the year 5769 into being. I will make several jokes referencing the fact that we still write 5758 on all of our checks. Just what are these celebrations I’m talking about? Today, millions of Jewish people around the world, including several of us in Grand Forks, are celebrating Rosh Hashanah—the Jewish new year.
   
Blog.Rabbi,Jason
MONDAY, JANUARY 07, 2013
Early Hanukkah in 2013: Jewish Calendar Fun
(...)
Several years ago David Letterman quipped in a Late Show monologue, “Happy Rosh Hashanah, it’s the Jewish new year and the year is 5768. I, uh, it’s funny I’m still writing 5767 on my checks.” Well, unlike Dave, most of us use the Gregorian calendar in our everyday lives, but as Jews we must be attuned to the Jewish calendar as well. It is the rhythm of our Jewish lives. Perhaps this year’s anomaly in the Jewish calendar will cause people to learn more about the lunar calendar that governs the Jewish year.