Fix time precision issue for long VIC runs (#668)

* Fixed time precision issue for long VIC runs

* Added release note for the time precision bug fix

* Minor release note update

Author: yixinmao

docs/Development/ReleaseNotes.md

@@ -46,10 +46,14 @@ To check which release of VIC you are running:
 
 	Before the fix, `dz_node` and `node_depth` in image driver output state file were not spatially distributed, which was wrong. Now these two variables are spatially distributed in the output state file.
 
-6. Fixed a bug related to `run_cell` and `mask` variables in image driver inputs ([GH#662]((https://github.com/UW-Hydro/VIC/pull/662)))
+8. Fixed a bug related to `run_cell` and `mask` variables in image driver inputs ([GH#662](https://github.com/UW-Hydro/VIC/pull/662))
 
 	Before the fix, active cell was controlled by `mask` variable in the domain file in image driver, and `run_cell` variable in the parameter file was not actually used. Now `run_cell` variable in the parameter file controls active cells (`run_cell` must be within the mask defined by the domain file).
 
+9. Fixed a time precision bug for long simulations ([GH#668](https://github.com/UW-Hydro/VIC/pull/668))
+
+	Before the fix, the timestamps of long VIC runs were incorrect in some cases due to precision issue in timestamp generation. This resulted in incorrect output timestamps after running for a long period of time, or output termination. Please refer to [GH#668](https://github.com/UW-Hydro/VIC/pull/668) for details on this bug fix.
+
 
 ------------------------------
 

vic/drivers/shared_all/src/make_dmy.c

@@ -43,8 +43,6 @@ make_dmy(global_param_struct *global)
     double                  dt_time_units, start_num, end_num, force_num,
                             numdate;
 
-    dt_seconds_to_time_units(global->time_units, global->dt, &dt_time_units);
-
     start_dmy.dayseconds = global->startsec;
     start_dmy.year = global->startyear;
     start_dmy.day = global->startday;
@@ -107,9 +105,10 @@ make_dmy(global_param_struct *global)
     temp = calloc(global->nrecs, sizeof(*temp));
 
     /** Create Date Structure for each Model Time Step **/
-    for (i = 0, numdate = start_num;
-         i < global->nrecs;
-         i++, numdate += dt_time_units) {
+    for (i = 0; i < global->nrecs; i++) {
+        dt_seconds_to_time_units(global->time_units, i * global->dt,
+                                 &dt_time_units);
+        numdate = start_num + dt_time_units;
         num2date(global->time_origin_num, numdate, 0., global->calendar,
                  global->time_units, &temp[i]);
     }