The Wikibase date datatype has a notion of precision, and also a lesser known notion of upper and lower bounds expressed as integer multiples of this precision.
For instance, the following data value represents the current day in UTC:
Timestamp +2018-05-17T00:00:00Z
Timezone +00:00
Calendar Gregorian
Precision 1 day
Before 0
After 1
(meaning that it starts at the beginning of 2018-05-17, midnight, and continues until the next day)
When creating a claim with a date value in the Wikibase UI, by default the "After" parameter is set to 0 (see [[ https://www.wikidata.org/w/index.php?title=Q4115189&diff=prev&oldid=680600484 |this example]]). Is it intentional? I believe that 1 should be the default value there (regardless of the precision).
If this is indeed a bug, should we run a bot to fix all the date values?
If this is not a bug, then there is a bug in Wikidata-Toolkit, which agrees with my interpretation of the default value:
https://github.com/Wikidata/Wikidata-Toolkit/blob/297160438fecb86e7b86ae05cee3c46eae44797d/wdtk-datamodel/src/main/java/org/wikidata/wdtk/datamodel/interfaces/TimeValue.java
(except that there is a typo there: before-tolerance should obviously be after-tolerance in the last comment)