This template allows you to specify its parameter as the sort key for a cell in sortable tables.

{{Hidden sort key|sort term}}
Insert this template with the appropriate sort term before the cell data.
{| class="wikitable sortable"
|-
!C1
|-
|{{Hidden sort key|1999-12-15}}15 December 1999
|-
|{{Hidden sort key|1997-04-21}}21 April 1997
|-
|{{Hidden sort key|1998-04-21}}21 April 1998
|}

produces:

C1
15 December 1999
21 April 1997
21 April 1998