If your mother passed away on March 18, 2004 then indeed the yahrzeit is observed in Adar I in leap years and Adar during ordinary years.

In fact, this is exactly what the Hebcal Yahrzeit + Anniversary calendar produces:

https://www.hebcal.com/yahrzeit?v=yahrzeit&y1=2004&m1=3&d1=18&t1=Yahrzeit

Where are you seeing different results for March 18, 2004?

Hi, thanks for using Hebcal.

If you'd like to delete Hebcal from your macOS calendar, you can follow these instructions:

https://www.hebcal.com/home/78/apple-ical-delete-hebcal-jewish-calendar

Hi Ms. Jackler, thanks for using Hebcal.

We checked our security certificate and we think it's perfectly fine.

https://www.sslshopper.com/ssl-checker.html#hostname=hebcal.com

Can you double-check your web browser and make sure you're running a recent version of Chrome, Safari, Firefox or Edge?

Thanks for using Hebcal!

If you'd like to display the Hebrew date in addition to the Gregorian date, please visit https://www.hebcal.com/hebcal and create a customized calendar and check one of these two boxes.

• Show Hebrew date for dates with some event
• Show Hebrew date every day of the year

Hi, thanks for using Hebcal.

We regret that we do not currently offer a personalized zmanim service (alot haShachar, mincha ketana, etc).

We do offer candle-lighting and havadalah times for holidays and fast/start end times via https://www.hebcal.com/hebcal

Hi, thanks for using Hebcal's triennial Torah Reading feature.

In December 2021 we updated our Triennial readings to reflect the November 2020 responsa from the CJLS which modified the 3rd year readings for a half-dozen parshiyot. We published a short note about this change on the Hebcal.com website here:

https://www.hebcal.com/home/3824/triennial-torah-reading-revision

Since you reference Yad LaTorah from 2002, this obviously was published before the November 2020 CJLS responsa. So Yad LaTorah was correct at the time it was published, but it has become out-of-date.

Hi, thanks for using the Hebcal Hebrew/Gregorian date converter.

https://www.hebcal.com/converter?gd=17&gm=3&gy=1968&g2h=1

Sun, 17 March 1968 (before sunset) = 17th of Adar, 5728

Hebrew year 5728 was not a leap year (note month: Adar).

Since Hebrew Year 5728 was not a leap year, our converter page displays "Adar" as there is no ambiguity about the month of Adar in a non-leap year.

Hi, thanks for writing to the Hebcal Jewish Calendar support page.

We don't conduct Shabbat services - we're just an online calendar resource.

Hebcal (pronounced HEEB-kal, as in Hebrew calendar) is a free Jewish calendar and holiday web site.

Our mission is to increase awareness of Jewish holidays and to help Jews to be observant of the mitzvot.

We offer a powerful custom Jewish calendar tool that lets you generate a list of Jewish holidays for any year (past, present or future). Also available are a Hebrew date converter, Shabbat candle lighting times and Torah readings (both full kriyah and triennial system), and a page to look up yahrzeits, birthdays and anniversaries.

Hi,

https://www.hebcal.com/converter?gd=29&gm=3&gy=1942&g2h=1

• Sun, 29 March 1942 = 11th of Nisan, 5702
• י״א בְּנִיסָן תש״ב

Since Passover is the 15th of Nisan, the parsha that week was Pesach.