diff --git a/guix-data-service/web/compare/html.scm b/guix-data-service/web/compare/html.scm
index cf04d3b..23a63c0 100644
--- a/guix-data-service/web/compare/html.scm
+++ b/guix-data-service/web/compare/html.scm
@@ -55,7 +55,7 @@
       #:font-family "monospace")
      (form-horizontal-control
       "Base datetime" query-parameters
-      #:help-text "The date and time to compare from."
+      #:help-text "The date and time to compare from. The required format is YYYY-MM-DD HH:MM:SS"
       #:font-family "monospace")
      (form-horizontal-control
       "Target branch" query-parameters
@@ -64,7 +64,7 @@
       #:font-family "monospace")
      (form-horizontal-control
       "Target datetime" query-parameters
-      #:help-text "The date and time to compare to."
+      #:help-text "The date and time to compare to. The required format is YYYY-MM-DD HH:MM:SS"
       #:font-family "monospace")))
    (else
     '())))
diff --git a/guix-data-service/web/query-parameters.scm b/guix-data-service/web/query-parameters.scm
index 4572ff3..6be4848 100644
--- a/guix-data-service/web/query-parameters.scm
+++ b/guix-data-service/web/query-parameters.scm
@@ -217,7 +217,7 @@
     (lambda (key . args)
       (make-invalid-query-parameter
        s
-       "Unable to parse date, please use YYYY-MM-DD HH-MM-SS as the format."))))
+       "Unable to parse date, please use YYYY-MM-DD HH:MM:SS as the format."))))
 
 (define (parse-checkbox-value s)
   (string=? s "on"))