I am sponsoring the fix for this as NMU.
Please find the changes in the attached debdiff.

diff -Nru calcurse-4.8.1/debian/changelog calcurse-4.8.1/debian/changelog
--- calcurse-4.8.1/debian/changelog 2023-11-01 11:45:27.000000000 +0100
+++ calcurse-4.8.1/debian/changelog 2025-03-05 22:14:01.000000000 +0100
@@ -1,3 +1,11 @@
+calcurse (4.8.1-1.2) unstable; urgency=medium
+
+ * Non-maintainer upload.
+ * Update B-D to tzdata-legacy to fix FTBFS (Closes: #1086259)
+ * Recommend tzdata-legacy for full functionality.
+
+ -- Andrew Bower <andrew@bower.uk> Wed, 05 Mar 2025 21:14:01 +0000
+
calcurse (4.8.1-1.1) unstable; urgency=medium

* Non-maintainer upload.
diff -Nru calcurse-4.8.1/debian/control calcurse-4.8.1/debian/control
--- calcurse-4.8.1/debian/control 2023-11-01 11:45:27.000000000 +0100
+++ calcurse-4.8.1/debian/control 2025-03-05 22:10:20.000000000 +0100
@@ -2,13 +2,14 @@
Section: utils
Priority: optional
Maintainer: Bartosz Fenski <fenio@debian.org>
-Build-Depends: debhelper (>= 10.0.0), libncurses-dev, tzdata
+Build-Depends: debhelper (>= 10.0.0), libncurses-dev, tzdata-legacy
Standards-Version: 4.5.0
Homepage: https://calcurse