libfapidxx [FEATURE]: support negative time shift

src/libs/libfapidxx/fapid_sff_modwid2shift.cc

@@ -28,11 +28,12 @@
  * 
  * REVISIONS and CHANGES 
  *  - 04/07/2014   V1.0   Thomas Forbriger
+ *  - 11/12/2019   V1.1   support negative time shift
  * 
  * ============================================================================
  */
 #define TF_FAPID_SFF_MODWID2SHIFT_CC_VERSION \
-  "TF_FAPID_SFF_MODWID2SHIFT_CC   V1.0   "
+  "TF_FAPID_SFF_MODWID2SHIFT_CC   V1.1"
 #define TF_FAPID_SFF_MODWID2SHIFT_CC_CVSID \
   "$Id: $"
 
@@ -65,7 +66,11 @@ int sff_modwid2shift__(char *wid2line, real *tmin, real *
 {
   fapidxx::WID2container wid2c(wid2line, wid2line_len);
   libtime::TAbsoluteTime date(wid2c.wid2.date);
-  wid2c.wid2.date+=libtime::double2time(double(*tsec)+60.*double(*tmin));
+  double tshift=double(*tsec)+60.*double(*tmin);
+  if (tshift < 0.)
+  { wid2c.wid2.date-=libtime::double2time(-tshift); }
+  else
+  { wid2c.wid2.date+=libtime::double2time(tshift); }
   wid2c.encode(wid2line, wid2line_len);
   return 0;
 } // int sff_modwid2shift__