In MainDlgProc create font for it:
...
static HFONT hFont;
...
case WM_INITDIALOG:
hFont = CreateFont(18, -18, 0, 0, 700, 0, 0, 0, 0, 0, 0, 0, 0, TEXT("Arial"));
SendMessage(GetDlgItem(hwndDlg, txt_Time), WM_SETFONT, (WPARAM)hFont, FALSE);
...
case WM_CLOSE:
...
DeleteObject(hFont);
...