Add date arithmetic to VDATE variables

[chhoumann]

Summary

Enable date arithmetic operations on VDATE variables to calculate relative dates (e.g., add/subtract days).

Problem

Currently, VDATE values are static once set:

• Cannot calculate dates relative to a VDATE value
• Must use multiple VDATE prompts for related dates
• No way to express "due date + 7 days" logic
• Limits complex date-based workflows

Use Cases

• Project management: Set end date, calculate milestone dates (end date - 10 days, end date - 5 days)
• Task scheduling: Set start date, auto-calculate checkpoints (+7 days, +14 days, +30 days)
• Recurring content: Base date + intervals for recurring tasks/notes
• Deadline tracking: Calculate warning dates before deadlines

Proposed Syntax Options

Option 1: Inline arithmetic

{{VDATE:projectEnd,YYYY-MM-DD}}
Milestone 1: {{VDATE:projectEnd-10,YYYY-MM-DD}}
Milestone 2: {{VDATE:projectEnd-5,YYYY-MM-DD}}
Final Due: {{VDATE:projectEnd+0,YYYY-MM-DD}}

Option 2: Reference with offset

{{VDATE:baseDate,YYYY-MM-DD}}
Check-in 1: {{VDATE+7:baseDate,YYYY-MM-DD}}
Check-in 2: {{VDATE+14:baseDate,YYYY-MM-DD}}

Features

• Add/subtract days: +7, -10
• Optional: weeks (+2w), months (+1m), years (+1y)
• Reference existing VDATE variables
• Calculate from today: {{VDATE+7:today,YYYY-MM-DD}}

Benefits

• Complex date workflows with single input
• Automated relative date calculations
• Reduced manual date entry errors
• More powerful project templates

Related

• Issue #69 requests VDATE reuse with different formatting
• This extends that concept with arithmetic operations