Daily avg = Annual / 365
Weekly avg = Annual / 52
Monthly avg = Annual / 12

<scale>
<offset>

<ComplexSource>

<VRTDataset rasterXSize="800" rasterYSize="800">
  <SRS>GEOGCS["WGS 84",DATUM["WGS_1984",SPHEROID["WGS 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],AUTHORITY["EPSG","6326"]],PRIMEM["Greenwich",0],UNIT["degree",0.0174532925199433],AUTHORITY["EPSG","4326"]]</SRS>
  <GeoTransform> -6.2000000000000000e+01,  2.5000000000000001e-03,  0.0000000000000000e+00,  1.5000000000000000e+01,  0.0000000000000000e+00, -2.5000000000000001e-03</GeoTransform>
  <Metadata>
    <MDI key="AREA_OR_POINT">Area</MDI>
  </Metadata>
  <VRTRasterBand dataType="Int16" band="1">
    <NoDataValue>nan</NoDataValue>
    <ColorInterp>Gray</ColorInterp>
    <scale>3.28084</scale>
    <SimpleSource>
      <SourceFilename relativeToVRT="1">DEM_meters.tif</SourceFilename>
      <SourceBand>1</SourceBand>
      <SourceProperties RasterXSize="800" RasterYSize="800" DataType="Int16" BlockXSize="800" BlockYSize="1" />
      <SrcRect xOff="0" yOff="0" xSize="800" ySize="800" />
      <DstRect xOff="0" yOff="0" xSize="800" ySize="800" />
    </SimpleSource>
  </VRTRasterBand>
</VRTDataset>

gdal_translate -a_scale ...