Tini gets spuriously rounded when saving

[dreeves]

Steps To Reproduce

1. At some odd time like 7:57pm, create a promise like alice.commitsto.review/test-by-8:03pm
2. Note that tini and tdue appear correct to the minute
3. Click SAVE

Expect

We expect tini and tdue to stay correct

WTF

They both jump forward to the next even half hour.

I have use cases where the exact time of both tdue and tini matters a lot so this feels like a high severity bug despite the errors involved being at most 29 minutes.