An SLA (Service Level Agreement) is an agreed-upon time period within which a response and/or resolution should be provided for a ticket.
Within Freshdesk, you can create an SLA policy under Admin > Workflows > SLA policies > Add Policy. You can set the SLA Targets and set rules for which this SLA will have to apply.
You also have the option to associate specific Business Hours with the SLA so that the SLA applies only for that business hour (time period).
When a ticket is newly created on the portal, the default status of this ticket is "open." So this is when the SLA timer begins on the ticket and the time gets calculated on the ticket. The response time and resolution time on a ticket are determined by the SLA policy applied to this ticket and the details of this could be checked in Admin ->Workflows -> SLA policies.
When you reply to a customer or wait for a third party to give you information, you could change the status to pending or "awaiting customer response." The SLA timer could be switched off for such statuses in Admin -> Workflows -> ticket fields -> click on the status dropdown to toggle off the respective times next to these statuses.
Go to the Tickets tab -> click on a ticket, in the Ticket details page, you'll find the 'Edit' option right above the Subject of the ticket.

Note:
1) The option to change the due date will only show up when the ticket is assigned to statuses that have the SLA timer ON (example: Open).
You can check which statuses have their SLA timers ON or OFF under Admin > Workflows > Ticket Fields > Status field. Once a manual change is done to the Due Time of a ticket, it will not change again when the ticket properties (for example, a change in priority) are updated.
2) The due by date and time can always be updated only to a value greater than the First response time, which is the 'Response due' time on the ticket.
When a ticket is created in your portal, you could set a priority to the ticket according to the subject and level of urgency expressed by the customer. In the SLA policy, you would be able to set a first response time as well as resolution time. Please navigate to Admin -> Workflows -> SLA policies -> click on edit next to the policy.
When the first response time is violated the ticket would contain a "response due" tag which could be seen when you see this ticket in the queue within your tickets list view.
The resolution time when violated would give the ticket a tag called "overdue" which can also be seen when you check your queue.
While configuring SLA, you would also have to option to set up escalation rules, which would send out notifications to the chosen agents, whenever the configured SLA is violated. This could be set up under Admin > Workflows > SLA Policies > Edit >"What happens when this SLA is violated?" where you could add multiple levels of escalation.
However, only agents(any agent within your Freshdesk Account, even from a different group) in your account could be added under this section and third parties could not be added.
You could configure Escalation Rules when an agent has not sent the first response within a set time. This could be done under the Admin > Workflows > SLA Policies >Edit >"What happens when this SLA is violated?"-->"Set escalation rule when a ticket is not responded to on time".
You could also configure a Supervisor Rule(under Admin > Workflows > Ticket Updates > New Rule). A sample rule would be
Conditions: Hours since First Response Due
Actions: Send email to agent-->Escalation Agent Name
You could configure Business Hours on your account, based on your active time period for each day of the week. This could be done from under Admin > Team > Business Hours > Edit and change under "Business Hours".
Once this is done, you could associate the SLA to work based on Business Hours.
You could add Holidays to your Freshdesk Business Calendar or remove specific dates from the Holiday Calendar.
Set Holidays:
Yearly holiday list can be configured by adding specific dates to the holiday calendar. This could be done from under Admin > Team > Business Hours > Edit > Holidays, where you can add a date to your holiday calendar.
Change calendar holidays:
Kindly navigate to Admin > Team > Business Hours > Edit > and under the Holidays section, you could remove a specific Holiday from the list.
Please navigate to Admin -> Account -> Helpdesk Settings to see the option to change the time zone.
Kindly change it with respect to your location and it would reflect in your portal.
As of now, Freshdesk does not have an option to have the Time Format to be available in a 24-hour format.