diff --git a/winpr/tools/makecert-cli/winpr-makecert.1.in b/winpr/tools/makecert-cli/winpr-makecert.1.in
index a50c82c49..a1eb841b7 100644
--- a/winpr/tools/makecert-cli/winpr-makecert.1.in
+++ b/winpr/tools/makecert-cli/winpr-makecert.1.in
@@ -72,10 +72,10 @@ can be thought as "dummy" mode.
 .IP "-n common_name"
 The common name to use in the certificate.
 .IP "-m months"
-Validity period in months.
+Validity period in months (multiple of 31 days, not clanendar months).
 .IP "-y years"
-Validity period in years. If months and years are specified the specified
-month parameter will take precedence.
+Validity period in years (365 days, leap years not accounted). If months and years are specified the specified
+the values are accumulated.
 .IP "-len length"
 Key length in bits to use.
 .IP "-a { \fImd5\fP | \fIsha1\fP | \fIsha256\fP | \fIs384\fP | \fIsha512\fP }"
diff --git a/winpr/tools/makecert/makecert.c b/winpr/tools/makecert/makecert.c
index 85baeef95..0929c1d56 100644
--- a/winpr/tools/makecert/makecert.c
+++ b/winpr/tools/makecert/makecert.c
@@ -405,7 +405,7 @@ static int makecert_context_parse_arguments(MAKECERT_CONTEXT* context,
 
 			val = strtol(arg->Value, NULL, 0);
 
-			if ((errno != 0) || (val < 1) || (val > 12))
+			if ((errno != 0) || (val < 0))
 				return -1;
 
 			context->duration_months = (int)val;
@@ -999,10 +999,9 @@ int makecert_context_process(MAKECERT_CONTEXT* context, int argc, char** argv)
 #endif
 		X509_gmtime_adj(before, 0);
 
-		if (context->duration_months)
-			X509_gmtime_adj(after, (long)(60 * 60 * 24 * 31 * context->duration_months));
-		else if (context->duration_years)
-			X509_gmtime_adj(after, (long)(60 * 60 * 24 * 365 * context->duration_years));
+		long duration = context->duration_months * 31l + context->duration_years * 365l;
+		duration *= 60l * 60l * 24l;
+		X509_gmtime_adj(after, duration);
 	}
 	X509_set_pubkey(context->x509, context->pkey);
 	name = X509_get_subject_name(context->x509);