Fix timezone-dependent tests

These tests fail depending on timezone and current time if the date
differs from the UTC date. This was remedied by using `date -u`, which
is POSIX.

test/preamble

@@ -41,31 +41,31 @@ end 0
 unset SOURCE_DATE_EPOCH
 
 begin "Writes the appropriate header"
-scdoc <<EOF | grep '^\.TH "test" "8" "'"$(date +'%F')"'"' >/dev/null
+scdoc <<EOF | grep '^\.TH "test" "8" "'"$(date -u +'%F')"'"' >/dev/null
 test(8)
 EOF
 end 0
 
 begin "Preserves dashes"
-scdoc <<EOF | grep '^\.TH "test-manual" "8" "'"$(date +'%F')"'"' >/dev/null
+scdoc <<EOF | grep '^\.TH "test-manual" "8" "'"$(date -u +'%F')"'"' >/dev/null
 test-manual(8)
 EOF
 end 0
 
 begin "Handles extra footer field"
-scdoc <<EOF | grep '^\.TH "test-manual" "8" "'"$(date +'%F')"'" "Footer"' >/dev/null
+scdoc <<EOF | grep '^\.TH "test-manual" "8" "'"$(date -u +'%F')"'" "Footer"' >/dev/null
 test-manual(8) "Footer"
 EOF
 end 0
 
 begin "Handles both extra fields"
-scdoc <<EOF | grep '^\.TH "test-manual" "8" "'"$(date +'%F')"'" "Footer" "Header"' >/dev/null
+scdoc <<EOF | grep '^\.TH "test-manual" "8" "'"$(date -u +'%F')"'" "Footer" "Header"' >/dev/null
 test-manual(8) "Footer" "Header"
 EOF
 end 0
 
 begin "Emits empty footer correctly"
-scdoc <<EOF | grep '^\.TH "test-manual" "8" "'"$(date +'%F')"'" "" "Header"' >/dev/null
+scdoc <<EOF | grep '^\.TH "test-manual" "8" "'"$(date -u +'%F')"'" "" "Header"' >/dev/null
 test-manual(8) "" "Header"
 EOF
 end 0