Replace src/date.cpp with C++20 <chrono> and <format>

Author: jwakely

Makefile

@@ -32,13 +32,13 @@ all: pgms
 
 pgms: $(PGMS)
 
-bin/lists: src/date.o src/issues.o src/status.o src/sections.o src/mailing_info.o src/report_generator.o src/lists.o src/metadata.o
+bin/lists: src/issues.o src/status.o src/sections.o src/mailing_info.o src/report_generator.o src/lists.o src/metadata.o
 
 bin/section_data: src/section_data.o
 
 bin/toc_diff: src/toc_diff.o
 
-bin/list_issues: src/date.o src/issues.o src/status.o src/sections.o src/list_issues.o src/metadata.o
+bin/list_issues: src/issues.o src/status.o src/sections.o src/list_issues.o src/metadata.o
 
 bin/set_status: src/set_status.o src/status.o