diff --git a/lib/parsedate.c b/lib/parsedate.c
index ef91585e5dd359b878b4a9ae95a0658a897dd35b..5dd6c6ef7228e362a2c11a9a2153d2cef5b31288 100644
--- a/lib/parsedate.c
+++ b/lib/parsedate.c
@@ -154,7 +154,7 @@ static const struct tzinfo tz[]= {
    0 monday - 6 sunday
 */
 
-static int checkday(char *check, size_t len)
+static int checkday(const char *check, size_t len)
 {
   int i;
   const char * const *what;
@@ -173,7 +173,7 @@ static int checkday(char *check, size_t len)
   return found?i:-1;
 }
 
-static int checkmonth(char *check)
+static int checkmonth(const char *check)
 {
   int i;
   const char * const *what;
@@ -193,7 +193,7 @@ static int checkmonth(char *check)
 /* return the time zone offset between GMT and the input one, in number
    of seconds or -1 if the timezone wasn't found/legal */
 
-static int checktz(char *check)
+static int checktz(const char *check)
 {
   unsigned int i;
   const struct tzinfo *what;