fix the conversion of fractional seconds to ticks

ticks are 1/10th of a second. Simeply taking the value after
the decimal point and treat it as seconds does not for work
for multi digit fractions (e.g. 50.52 would add 52 ticks when only
5 ticks where correct).
when converting strings to ticks, try to parse the string as
RFC 3339 timestamp first, fall back to plain integer next.