diff --git a/jquery-ui-timepicker-addon.js b/jquery-ui-timepicker-addon.js
index 75f0825..1263f0f 100644
--- a/jquery-ui-timepicker-addon.js
+++ b/jquery-ui-timepicker-addon.js
@@ -610,8 +610,8 @@ $.datepicker._doKeyPress = function(event) {
 		if ($.datepicker._get(inst, 'constrainInput')) {
 			var datetimeChars = tp_inst._defaults.timeFormat.toString()
 								.replace(/[hms]/g, '')
-								.replace(/TT|T/g, 'APM')
-								.replace(/tt|t/g, 'apm') +
+								.replace(/TT|T/g, tp_inst._defaults.ampm ? 'APM' : '')
+								.replace(/tt|t/g, tp_inst._defaults.ampm ? 'apm' : '') +
 								" " +
 								$.datepicker._possibleChars($.datepicker._get(inst, 'dateFormat')),
 				chr = String.fromCharCode(event.charCode === undefined ? event.keyCode : event.charCode);