A long archive and a long connected history are different descriptions. If email begins in 2018, CRM events in 2023 and support events in 2025, their shared period cannot begin before 2025. Calculate overlap from observed dates, then inspect references to see whether records in that period describe the same work.
Work the dates explicitly
The following dates are fictional. Email spans January 1, 2018 to September 30, 2026. CRM history spans July 1, 2023 to October 5, 2026. Support events span March 1, 2025 to October 1, 2026. The common interval begins March 1, 2025 and ends September 30, 2026.
The latest start limits how far back all three groups coexist. The earliest end limits how far forward all three coexist. This calculation uses calendar boundaries; it does not count events or establish that coverage is continuous.
| Fictional group | Observed start | Observed end |
|---|---|---|
| 2018-01-01 | 2026-09-30 | |
| CRM history | 2023-07-01 | 2026-10-05 |
| Support events | 2025-03-01 | 2026-10-01 |
| Shared interval | 2025-03-01 | 2026-09-30 |
Handle unknown dates and empty overlap
If one group’s start or end is unknown, leave the combined interval unresolved rather than filling it with the company’s founding date. If the latest start falls after the earliest end, there is no common interval for the selected groups. You can still describe separate collections or choose a subset that overlaps.
Check what the dates actually mean
An earliest record date establishes that one early record exists. It does not prove every intervening month is retained. A later migration might preserve documents while dropping activities. Inspect a few points across the period and note observed gaps.
Also distinguish event dates from export dates and file-modified dates. A file exported today can contain work from years ago. A recently modified file can refer to an older project. Choose the date field that matches the inventory question.
Overlap is a time test, not a join test
Two systems can cover the same year while recording unrelated customers or departments. Check a shared work identifier, explicit cross-reference or documented relationship. If the link is only similar names and dates, record it as a candidate to inspect rather than a verified connection.
- Record the date field used for each range.
- Keep each group’s original range.
- Calculate latest start and earliest end.
- Mark gaps or migrations within the range.
- Inspect identifiers on a representative shared event.
- Retain older groups as separate collections where useful.
Use the result to narrow the next task
A shared interval can help you choose a practical period for inspection. For example, inspect a single quarter within the overlap before preparing a broader sample. The purpose is to understand the collection, not to declare older records worthless or assign a payout based on years.
Questions about this comparison
Do account creation dates establish history?
No. Use dates observed in the retained records or exports.
Does an overlapping year mean records can be joined?
No. Dates establish temporal overlap. Identifiers and references establish relationships.
What if the ranges touch on one day?
The boundary dates overlap, but you still need to check whether actual events exist on that day and what time conventions apply.