If you need to calculate a new value, you should generally strive to do it in a search list extension or XType. However, there are occasions where it is quicker and easier to do the calculation right in the template, using the Cheetah directive #set.

Here's an example where we calculate the elapsed time between dawn and dusk:

#set $dawntodusk = $almanac(horizon=-6).sun(use_center=1).set.raw - $almanac(horizon=-6).sun(use_center=1).rise.raw

The expression calculates the simple difference between two values: the time of the sunset minus the time of the sunrise, both in unix epoch time. The result will be the elapsed time in seconds. For details on the $almanac tag, see Almanac. For details on the .raw suffix, see Formatting options.

We can make use of this tag by simply referencing $dawntodusk. For example:

<p>The elapsed time between dawn and dusk is $dawntodusk</p>

This will result in something like

The elapsed time between dawn to dusk is 57315.0023124218

Not very elegant.

To get something more readable, while having the results honor any user-specified settings, including unit conversions and formatting, is a bit more involved. Here's a more sophisticated version that leverages classes ValueTuple and ValueHelper to do the formatting. While this example does not involve any unit conversions, in general, these classes can take care of that as well.

#from weewx.units import ValueTuple, ValueHelper                                                                   ## 1
#set $dawn = $almanac(horizon=-6).sun(use_center=1).rise.raw                                                       ## 2
#set $dusk = $almanac(horizon=-6).sun(use_center=1).set.raw                                                        ## 3
#set $dawntodusk = $dusk - $dawn                                                                                   ## 4
#set $dawntodusk_vh = ValueHelper(ValueTuple($dawntodusk,'second','group_deltatime'),formatter=$station.formatter) ## 5

The numbers reference the comments below:

1. Neither class ValueTuple, nor ValueHelper, appear in the template. We must import them.

2. Set the value $dawn to the time of sunrise. The .raw suffix ensures that the results will be in unix epoch time (rather than a string).

3. Similar, but with sunset.

4. Take the difference. Because unix epoch time is in seconds, the results will also be in seconds.

5. Construct a ValueHelper to hold the results. We borrow a formatter from the $station tag.

The results, $dawntodusk_vh, is a ValueHelper. When rendered as a string by the Cheetah engine, it will do the necessary magic to ensure sensible formatting for the ValueTuple that it holds internally. This time, the template

<p>The elapsed time between dawn and dusk is $dawntodusk_vh.</p>

gives the following results:

The elapsed time between dawn and dusk is 0 days, 15 hours, 55 minutes

Like any other instance of ValueHelper, you can override the formatting. For example, specifying

The elapsed time between dawn and dusk is
    $dawntodusk_vh.format("%(hour)d%(hour_label)s, %(minute)d%(minute_label)s, and %(second)d%(second_label)s")

gives the results

The elapsed time between dawn and dusk is 15 hours, 55 minutes, and 15 seconds