Low-precision times should zero unspecified fields

[hollasch]

Here's an example:

> timeprint --now --time 20181029T160000 %_Dd %_d0H:%_h0M:%_m0S
3d 05:13:30

> timeprint --now --time 20181029T1600 %_Dd %_d0H:%_h0M:%_m0S
3d 05:14:00

Notice that if the time here is specified as 1600, then the delta time uses the same precision for --now, zeroing out the seconds. In this situation, a time of 1600 (or even 16) should be interpreted as 160000.