Validate format for dateFormat and addToDate method #22
Description
Is your feature request related to a problem? Please describe.
dateFormat and addToDate function does not validate the dateFormat supplied as input to the function and accepts any String.
Describe the solution you'd like
Restrict access to only valid dateFormats supported by SimpleDateFormat. The validation should be compile-time rather than at runtime.
Additional context
See Xtext Validations for pointers. You will either need to change the grammar or add custom validation.