With modern Cams (and of course the LRTimelapse ProTimer ;-)) intervals with parts of a second become more usual. I think timestamps in the exif as basic are only in full seconds but what about a check if the intervals in a sequence isn't constant linked with a short explanation and the question whether this should be ignored or if LRTimelapse should calculate the subsecond-interval by dividing the time-difference from the last to first picture by the amount of pictures -1?

After shooting a moonrise with 0.5 s interval and two beer, so no idea if this idea is bullshit or not ;-)

Modern Cameras provide fractions of seconds in the exif data and those are also displayed in LRT and the intervals are being shown in 0.1 steps. Which camera are you using?

O.K. - so no need for this workaround any more.
A7R3 - interval was shown with 0 and 1 s alternating. Is it "DateTime(Original)" in the exif? Only full seconds, also with the newer A7s3 - yeah Sony, no Nikon I know ;-)

Should work with those files. For older sequences processed with former LRT versions try doing "Metadata / Reload Exif Data".

A7sIII works - perfect :-)