Your display is NOT a time, it is a string of minutes and seconds, a time has hours.
I would suggest converting it to seconds:
Try:
whileprintingrecords;
numbervar DecTime:={table.field};
numberVar dur := Totalsecs:=(DecTime*60);
numberVar hrs;
numberVar min;
numberVar sec;
stringVar hhmmss;
hrs := Truncate(Truncate(dur/60)/60);
min := Remainder(Truncate(dur/60),60);
sec := Remainder(dur,60);
hhmmss := totext(hrs, "0") + ":" + totext(min, "00") + ":" + totext(sec, "00");
hhmmss
-k