While Time Models are used when exploring data to see various offsets from the current date, Time Functions are an optional component of metrics in One Model. They are defined when creating metrics and they always apply to the metric, regardless of how it's viewed in explore.
While they're two different things, Time Functions work in combination with Time Models to create robust methods of exploring data across time. Only one time function can be applied to any Metric.
This article does not cover Creating Metrics, but that process is covered in the linked article. The only thing we'll call out here is that if you create a metric with a Time Function applied, you should name it accordingly so it's clear. An example would be Hires (Projected). On the metrics creation screen in One Model, the Time Function configuration can be found near the bottom of the settings. The following are the options and high-level explanations of each:
Rolling Months (and number) - Rolling Months is months going backward, to the day. Suppose today's date is September 10, 2019. If I create a metric with a time function of 12 rolling months and view it for today, it will sum the data going back to September 10, 2018 If I view the same metric by completed year, it will display the same values as a regular metric would, since it's going back 12 months from the end of each selected period (year).
Time Projected - This option projects the value for the selected time period based on the number as of the current point in that time period. If today is the 15th of the month, it will divide the number by 15 and multiply by the number of days in the month, quarter, or year for the projected value. For previous time periods it displays the actual value since that time period is complete. Note that Time Projected only works when viewing data by month, quarter, or year. It does not work for day.
Previous Period (and To Date) - Previous Period is just that. If you're viewing a previous period metric by month in explore, it'll display the number from the previous month. The same goes for year, quarter, and day. Calculated metrics displaying Difference From Previous Period can be created by subtracting a Previous Period metric from the corresponding metric with no time function applied.
Same Period Last Year (and To Date) - This is the same time period as what's selected in Explore, only for the prior year. The To Date option only impacts the current time period. For example, if today is September 10, 2019 and you have selected 2019, then Same Period Last Year is the total for all of 2018 but Same Period Last Year To Date is January 1, 2018 through September 10, 2018.
Year To Date (and Annualise) - Year to Date is the number for the past time for the year. When looking at a yearly basis it will be the YTD through the current date, but when looking at it a Quarterly, Monthly or Daily basis it will be the YTD through the period selected. Note that the Year to Date Time Function is meant to allow users to easily compare YTD this year to prior years. At the beginning of the year, Last Month or Last Quarter will not work because those time periods do not exist in the current year yet.
If Annualise is selected, it estimates the number based on the reporting period selected. Given that it's annualising based on that point in time, the annualization may not equal the actual number of hires for the year.
The following chart displays examples of metrics created with each Time Function. Compare the values for each row with the first (Hires) row to better understand each function.