filter_mod default for init_time

Note on this,
The default for init_time_days|seconds is 0,0

https://github.com/NCAR/DART/blob/4c89e64191dd905c58baf8a0d726c1e5ae24ceb6/assimilation_code/modules/assimilation/filter_mod.f90#L170-L173

So you have to give
filter_nml
init_time_days = -1
to read time from a restart file

The check in filter_set_inital_time is on only on days >=0 not on both namelist inputs as the comment suggests:
https://github.com/NCAR/DART/blob/4c89e64191dd905c58baf8a0d726c1e5ae24ceb6/assimilation_code/modules/assimilation/filter_mod.f90#L549-L550

https://github.com/NCAR/DART/blob/4c89e64191dd905c58baf8a0d726c1e5ae24ceb6/assimilation_code/modules/assimilation/filter_mod.f90#L1522-L1536

So not giving init_time_days

filter_nml
init_time_seconds = -1

ERROR FROM:
  source : time_manager_mod.f90
  routine: set_time
  message:  seconds, days are           -1           0  cannot be negative

https://docs.dart.ucar.edu/en/latest/assimilation_code/modules/assimilation/filter_mod.html#module-filter-mod

+------------------------------+---------------------+-------------------------------------------+
| init_time_days               | integer             | If negative, use the initial days read    |
|                              |                     | from the state data restart file.         |
|                              |                     | If positive, override the initial days    |
|                              |                     | read from state data restart files.       |
|                              |                     | Days since 1 Jan 1601.                    |
+------------------------------+---------------------+-------------------------------------------+
| init_time_seconds            | integer             | If negative use the initial seconds read  |
|                              |                     | from the state data restart file.         |
|                              |                     | If positive, override the initial seconds |
|                              |                     | read from state data restart files.       |
|                              |                     | Seconds since midnight.                   |
+------------------------------+---------------------+-------------------------------------------+

If you only give X seconds, the time is set to (0 days, X seconds)
not (model days, X seconds)

init_time_days is controlling the true|false from read_time_from_file and days, seconds can not be set independently.

Start in assimilation_code/modules/assimilation/filter_mod.f90, comparing the init_time_days and init_time_seconds defaults with filter_set_inital_time and the filter_nml handling around the referenced lines. Check the documented behavior in the filter_mod documentation and verify restart-file time handling for negative and partially specified values; done means the implementation, error behavior, and documentation agree.