Start MPAS dycore clock at current date on restarts

doc/ChangeLog

@@ -1,5 +1,64 @@
 ===============================================================
 
+Tag name:
+Originator(s): nusbaume
+Date:
+One-line Summary: Start the MPAS dycore clock at the current date on restarts
+Github PR URL:
+
+Purpose of changes (include the issue number and title text for each relevant GitHub issue):
+
+  Fixes #1721: MPAS dycore timer resets for restart runs
+
+  The MPAS dycore clock was always started at RUN_STARTDATE, so on
+  continue runs the MPAS time-step diagnostics in atm.log (and the MPAS
+  "xtime" field) were reset to the run start date. The MPAS clock now
+  starts at CAM's current date, which is the run start date for initial
+  runs and the restart date for continue and branch runs.
+
+  This change was developed with AI assistance (Claude Code).
+
+Describe any changes made to build system: N/A
+
+Describe any changes made to the namelist: N/A
+
+List any changes to the defaults for the boundary datasets: N/A
+
+Describe any substantial timing or memory changes: N/A
+
+Code reviewed by:
+
+List all files eliminated: N/A
+
+List all files added and what they do: N/A
+
+List all existing files that have been modified, and describe the changes:
+
+M  src/dynamics/mpas/dyn_comp.F90
+  - Set MPAS config_start_time from get_curr_date instead of get_start_date
+  - Set MPAS config_run_duration to 'none' so the MPAS clock stop time
+    comes from config_stop_time
+
+If there were any failures reported from running test_driver.sh on any test
+platform, and checkin with these failures has been OK'd by the gatekeeper,
+then copy the lines from the td.*.status files for the failed tests to the
+appropriate machine below.  All failed tests must be justified.
+
+derecho/intel/aux_cam:
+
+derecho/nvhpc/aux_cam:
+
+izumi/nag/aux_cam:
+
+izumi/gnu/aux_cam:
+
+CAM tag used for the baseline comparison tests if different than previous
+tag:
+
+Summarize any changes to answers: bit-for-bit unchanged (expected)
+
+===============================================================
+
 Tag name: cam6_4_213
 Originator(s): peverwhee, billsacks
 Date: Oct 16, 2026

src/dynamics/mpas/dyn_comp.F90

@@ -28,7 +28,7 @@ module dyn_comp
 use pio,                only: file_desc_t
 use cam_pio_utils,      only: clean_iodesc_list
 
-use time_manager,       only: get_start_date, get_stop_date, get_run_duration, &
+use time_manager,       only: get_curr_date, get_stop_date, &
                               timemgr_get_calendar_cf, get_step_size
 
 use cam_logfile,        only: iulog
@@ -273,7 +273,7 @@ subroutine dyn_readnl(NLFileName)
 
    ! Local variables
    integer, dimension(2) :: logUnits   ! stdout and stderr for MPAS logging
-   integer :: yr, mon, day, tod, ndate, nday, nsec
+   integer :: yr, mon, day, tod, ndate
    character(len=*), parameter :: subname = 'dyn_comp:dyn_readnl'
    !----------------------------------------------------------------------------
 
@@ -285,17 +285,19 @@ subroutine dyn_readnl(NLFileName)
    ! read namelist
    call cam_mpas_namelist_read(NLFileName, domain_ptr % configs)
 
-   ! Set config_start_date, etc. (these will not appear in the dycore namelist)
-   call get_start_date(yr, mon, day, tod)
+   ! Set config_start_time, etc. (these will not appear in the dycore namelist)
+   ! The MPAS clock starts at CAM's current time, which is the run start date for
+   ! initial runs and the restart date for continuation and branch runs.
+   call get_curr_date(yr, mon, day, tod)
    ndate = yr*10000 + mon*100 + day
    call mpas_pool_add_config(domain_ptr % configs, 'config_start_time', date2yyyymmdd(ndate)//'_'//sec2hms(tod))
 
    call get_stop_date(yr, mon, day, tod)
    ndate = yr*10000 + mon*100 + day
    call mpas_pool_add_config(domain_ptr % configs, 'config_stop_time', date2yyyymmdd(ndate)//'_'//sec2hms(tod))
 
-   call get_run_duration(nday, nsec)
-   call mpas_pool_add_config(domain_ptr % configs, 'config_run_duration', trim(int2str(nday))//'_'//sec2hms(nsec))
+   ! Let the MPAS clock use config_stop_time to set its stop time.
+   call mpas_pool_add_config(domain_ptr % configs, 'config_run_duration', 'none')
 
    ! Although the following namelist options are not expected to be used by CAM-MPAS, the MPAS-A dycore
    ! references these options, and they therefore must be defined in the configs pool