Invalid parameter when attempting to view the previous or next day in the calendar

As discovered by benkahn in MDLQA-14094:

Steps to reproduce:

1. Log in as a user and go to the calendar
2. Select 'Day' in the dropdown menu then click the link for the previous day

Expected result: The previous day is displayed

Actual result:  Invalid parameter value detected error (twice)

Note that the invalid parameter error is also sometimes displayed when clicking the link for the next day.

Invalid parameter value detected
File: /lib/externallib.php
Line: 332

Stack trace:

Missing required key in single structure: year
Error code: invalidparameter

• line 332 of /lib/externallib.php: invalid_parameter_exception thrown
• line ? of unknownfile: call to external_api::validate_parameters()
• line 223 of /lib/externallib.php: call to call_user_func()
• line 79 of /lib/ajax/service.php: call to external_api::call_external_function()
  Missing required key in single structure: year
  Error code: invalidparameter
• line 332 of /lib/externallib.php: invalid_parameter_exception thrown
• line ? of unknownfile: call to external_api::validate_parameters()
• line 223 of /lib/externallib.php: call to call_user_func()
• line 79 of /lib/ajax/service.php: call to external_api::call_external_function()

Invalid parameter value detected
File: /lib/externallib.php
Line: 332

Stack trace:

Missing required key in single structure: year
Error code: invalidparameter

• line 332 of /lib/externallib.php: invalid_parameter_exception thrown
• line ? of unknownfile: call to external_api::validate_parameters()
• line 223 of /lib/externallib.php: call to call_user_func()
• line 79 of /lib/ajax/service.php: call to external_api::call_external_function()