Since GEDCOM v7.0 the datatype DateValue is supposed to have an optional subtag PHRASE.

Examples:

1 BIRT
2 DATE 11 MAR 1925 
3 PHRASE 民國14年(歲次乙丑)2月17日

1 DEAT
2 DATE 29 MAR 1964
3 PHRASE Easter Sunday of 1964

1 EVEN
2 TYPE custom
2 DATE
3 PHRASE 5 January (year unknown)

This subtag is not yet supported by webtrees. I'm working on the required code changes.