diff --git a/modules/zcbor/CMakeLists.txt b/modules/zcbor/CMakeLists.txt
index 33b60bdfbc..45535fc859 100644
--- a/modules/zcbor/CMakeLists.txt
+++ b/modules/zcbor/CMakeLists.txt
@@ -10,6 +10,8 @@ if(CONFIG_ZCBOR)
     ${ZEPHYR_ZCBOR_MODULE_DIR}/src/zcbor_encode.c
   )
 
+  zephyr_library_compile_definitions(_POSIX_C_SOURCE=200809L)
+
   zephyr_compile_definitions_ifdef(CONFIG_ZCBOR_CANONICAL ZCBOR_CANONICAL)
   zephyr_compile_definitions_ifdef(CONFIG_ZCBOR_STOP_ON_ERROR ZCBOR_STOP_ON_ERROR)
   zephyr_compile_definitions_ifdef(CONFIG_ZCBOR_VERBOSE ZCBOR_VERBOSE)